Perforating Gun Assembly and Method of Use in Hydraulic Fracturing Applications The present invention relates to a perforating gun assembly and a method of use in hydraulic fracturing applications. In particular, the invention relates to a perforating gun assembly comprising convergent shaped charge orientations and a method of use in a multiple stage perforation operation prior to fracture initiation. Aspects of the invention relate to hydraulic fracturing methods. Background to the invention In the hydrocarbon exploration and production industry, it is common to use perforating guns to form fluid communication paths ("perforations") in a subterranean formation between a hydrocarbon reservoir and a drilled wellbore that traverses the reservoir. The communication paths enable the inflow of production fluids into the wellbore, and enable the delivery of stimulation fluids to the formation, for example during hydraulic fracturing operations. Typically perforation methods are applied to cased hole wellbores, which include a casing string cemented within the wellbore to increase the integrity of the wellbore and provide a flow path to surface for fluids produced from the formation. The perforations extend through the steel casing, the cement on the outside of the casing, and into the formation. Similar methods are used in the fields of water and geothermal exploration. It is conventional to form the perforations by placing a perforating gun which incorporates shaped charges inside the casing string next to the formation to be perforated. A typical perforation gun comprises a charge carrier and a series of shaped charges connected to a detonator by a detonation cord. The perforation gun forms a part of a tool string which is conveyed into the wellbore by a flexible line, drill string, coiled tubing, or other conveyance. Commonly, flexible line such as wireline, electric line or slickline is used to convey the perforating gun to the required wellbore depth. With the charge carriers located in the interval to be perforated, the shaped charges are detonated to generate high-pressure streams of particles in the form of jets. The jets penetrate through the casing, the cement and into the formation. The shaped charge weights, design, and orientation determine the size, depth of penetration and direction of the resulting perforation geometry. Various factors contribute to the effect of the perforations on the productivity of the well or the success of a fracturing operation. These include depth and effective diameter of perforation tunnels. One technique for generating perforations with improved inflow characteristics is to use groups or banks of convergent or focused shaped charges.

US 3,347,314, US 7,303,017, US 7, 172,023 and US 7,409,992 are examples of perforation devices which used convergent charge groups to create an enhanced perforation cavity. The cavities formed by the described methods are said to be of relatively large volume with high permeability, in order to enhance productivity. In hydraulic fracturing applications, it is necessary to pump fracturing fluid into the formation at a flow rate and pressure that is high enough to initiate fractures and cause them to propagate in the formation. This requires delivery of large volumes of fracturing fluids to the wellbore perforation sites. Flow of fluid through cased hole perforations creates perforation friction pressure which is dependent on parameters including flow rate, fluid viscosity and the size of holes in the wellbore casing. For a given cased wellbore, the perforation friction pressure is objectively reduced, through optimisation of the perforation design, so as to reduce the unwanted excessive pressure loss created by the perforations. Typically, perforation friction losses in the region of 100psi (689 kPa) are sought. The requirement to provide the desired flow rate for effective fracture initiation and propagation while staying within the designed perforation friction pressure will determine the nature of the intended perforation geometry, including parameters such as size and shot density of the perforations. Large diameter perforations from so-called "big hole" or large hole perforating guns enable higher flow rates for a given perforation friction pressure, and therefore reduce the requirement for longer perforated intervals. They also reduce the need for high pressure and high power pumps at surface, and mitigate the requirements to use high-grade materials and components in the wellbore completion. However, large hole perforations have limited penetration depths and result in inadequate control of the location of fracture initiation points and fracture propagation. It has been found that the perforations tend to act as competing fluid pathways during pumping of fracturing fluid. A consequence is that not all perforations will become fracture initiation sites. In contrast, small diameter perforations from high energy "deep hole" perforators will in general provide a greater depth of penetration (beyond the near-well damaged zone and the hoop-stress effected region around the wellbore), but in order to address the large pressure drop across the relatively small casing holes it is necessary to provide a larger number of wellbore perforations with a relatively high shot density. While conventional perforation techniques are capable of providing perforation geometries with relatively high density and adequate cumulative flow rate within the constraints of perforation friction pressure, the greater number of perforations increases the risk of multiple fracture initiation, and greater degree of growth tortuosity. Consequences include an increase in breakdown or treatment pressure and less effective fracture flow performance with reduced fracture connectivity, and a significant increase in early (premature) screen-out, meaning the fracture process comes to a halt well before the intended treatment objective has been reached. Hydra-jetting is a technique used as an alternative to ballistic perforating gun methods, and involves the use of hydra-jet tools deployed on coiled tubing to deliver high pressure fluid to the perforation location. The hydra-jet tool is capable of cutting a large diameter entry-hole (typically 1 to 2 inches or 25mm to 50mm) through the casing, and then creating a well-defined cavity in the formation outside the casing by removing cement and rock with the high pressure fluid. The resulting cavity provides an effective connection with the formation for fracture initiation and propagation, but hydra-jetting operations have certain disadvantages with respect to conventional ballistic perforation methods. Firstly, the equipment required for a hydra-jetting operation is complex and expensive. Secondly, the time required to form the hole and the cavity by hydra-jetting is significantly longer than the time required to form perforations using perforating guns, making the operation time- consuming and expensive. In addition, there are increased operational risks associated with hydra-jetting methods. Therefore, for operational and commercial reasons, various proposals have been made to provide improved perforation characteristics using ballistic perforation techniques rather than hydra-jetting. US 3,695,368 discloses a perforation method for production enhancement which uses a perforator with one or more sets of longitudinally spaced charges. After a first perforation is formed, the tool is axially repositioned before detonation of a second charge. The second charge forms a perforation contiguous with the first perforation. US 2004/0060734 describes a tandem oil well casing perforator which is intended to create a perforation with both big hole and deep hole characteristics for improved in-flow of produced hydrocarbons. The tandem perforator device uses a combination of a linear cutting charge and a hollow liner shaped charge at the same location. US 2007/158109 describes a perforation method which uses a combination of big hole and deep hole charges to create a fracture initiation site. The deep hole perforations are arranged to converge to a point beyond the principal axis of the big hole perforation. RU 2473788 describes an alternative method of forming a wellbore perforation in a two- stage process for improved in-flow of production fluids. In a first perforation cycle, either deep hole or big hole perforations are formed by detonation of deep hole or big hole shaped charges at a particular wellbore location. The perforation tool is removed and reloaded with deep hole or big hole charges (which may be the same or different from the charges used in the first perforation cycle). The reloaded gun is run into hole and in a second perforation cycle, the charges are detonated at the same wellbore location. WO2013/019390 describes a fracturing method which uses a tool with multiple convergent charge groups to create discrete fracture initiation sites at distinct locations, by

repositioning the tool between detonations. US2014/0020896 describes a method and apparatus for conducting multiple successive firings of perforation guns at a particular location in an openhole wellbore. The described method places a gun with n shaped charges at a particular location in the openhole and detonates the charges to create openings which penetrate the formation. The gun is retrieved and a reloaded (or second) gun is run to the same location to align n-1 charges with the openings. The method is repeated to provide relatively deep perforations which bypass the near-wellbore damaged formation and which are stated to be suitable for fracture initiation. Summary of the invention It is amongst the aims and objects of the invention to provide a perforating gun assembly and a method of use in hydraulic fracturing applications which is an alternative to the methods and apparatus described above. It is amongst the aims and objects of the invention to provide a perforating gun assembly and a method of use in hydraulic fracturing applications which obviates or mitigates drawbacks and deficiencies of previously proposed apparatus and methods. It is amongst the aims and objects of the invention to provide a multiple stage perforating gun assembly and a method of use in hydraulic fracturing applications which provides improved perforation connectivity and fracture initiation, while mitigating a requirement for unnecessarily high pumping pressures during fracturing. An aim of an aspect and embodiment of the invention is to provide a perforating gun assembly and a method of use in hydraulic fracturing applications which provides discrete fracture initiation sites with improved placement control. An aim of an aspect and embodiment of the invention is to provide a perforating gun assembly and a method of use in hydraulic fracturing applications with improved hydraulic efficiency. Additional aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided a method of perforating a cased or lined wellbore in a hydraulic fracturing operation, the method comprising:

- providing a perforating gun assembly comprising a first gun module on a first

longitudinal portion of the assembly and a second gun module on a second longitudinal portion of the assembly, wherein the first gun module comprises at least one large hole shaped charge, and wherein the second gun module comprises at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another;

- locating the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location;

- detonating the at least one large hole shaped charge of the first gun module to form one or more large holes in the casing or liner;

- translating the perforating gun assembly in the wellbore to locate the second gun module adjacent the casing or liner at the perforation location such that the position of the deep penetrating shaped charges corresponds to the position of the one or more large holes; and

- detonating the deep penetrating shaped charges of the second gun module to

generate jets which pass through the one or more large holes and form at least two deep penetration holes in the formation adjacent the perforation location, wherein the at least two deep penetration holes intersect to provide a connected channel in the formation. According to a second aspect of the invention, there is provided a method of perforating and fracturing a cased or lined wellbore, the method comprising:

- providing a perforating gun assembly comprising a first gun module on a first

longitudinal portion of the assembly and a second gun module on a second longitudinal portion of the assembly, wherein the first gun module comprises at least one large hole shaped charge, and wherein the second gun module comprises at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another;

- locating the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location;

- detonating the at least one large hole shaped charge of the first gun module to form one or more large holes in the casing or liner; - translating the perforating gun assembly in the wellbore to locate the second gun module adjacent the casing or liner at the perforation location such that the position of the deep penetrating shaped charges corresponds to the position of the one or more large holes;

- detonating the deep penetrating shaped charges of the second gun module to

generate jets which pass through the one or more large holes and form at least two deep penetration holes in the formation adjacent the perforation location, wherein the at least two deep penetration holes intersect to provide a connected channel in the formation;

- pumping a fracturing fluid into the wellbore to the perforation location and into the connected channel; and

- initiating and propagating a fracture from the connected channel. By providing lower and upper gun modules and two separate but co-operating detonation events, the charge or charges of the first (lower) gun module may be optimised to provide one or more large diameter casing entry holes, without concern about resulting

compromises to the penetration depth. The charges of the second (upper) gun module may be optimised to provide deep penetrating holes in the formation, without concern about resulting compromises to the casing entry size. Together, the first and second gun modules create perforations with desired perforation depth for a subsequent fracturing treatment, with the desired hydraulic performance. The terms "upper", "lower", "above", "below", "up" and "down" are used herein to indicate relative positions in the wellbore. The invention also has applications in wells that are deviated or horizontal, and when these terms are applied to such wells, they may indicate "left", "right" or other relative positions in the context of the orientation of the well. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa. According to a third aspect of the invention, there is provided a perforating gun assembly comprising:

a first gun module on a first longitudinal portion of the assembly, the first gun module comprising at least one large hole shaped charge; a second gun module on a second longitudinal portion of the assembly, the second gun module comprising at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another;

a first detonator for the at least one large hole shaped charge of the first gun module to form one or more large holes in the casing or liner;

a second detonator for the deep penetrating shaped charges of the second gun module to form one or more deep penetration holes in the casing or liner;

an anchor for securing the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location; and

a positioning and alignment module configured to translate the perforating gun assembly in the wellbore to locate the second gun module adjacent the casing or liner at the perforation location such that the position of the deep penetrating shaped charges corresponds to the position of the one or more large holes. Preferably the second gun module is configured to form at least two deep penetration holes that intersect to provide a connected channel in the formation. The second gun module may comprise at least one group of three or more deep penetrating shaped charges. A group of three or more deep penetrating shaped charges may all be arranged to generate jets oriented substantially along respective axes that converge towards one another. Alternatively, one or more of the charges in a group may be oriented on a non-convergent axis. Preferably, the first gun module comprises at least two large hole shaped charges.

Preferably, the first gun module comprises at least one group of large hole shaped charges. The group of large hole shaped charges may correspond with a group of deep penetrating shaped charges, and may have the same number of charges. Thus each charge in the second gun module may correspond to a charge in the first group of charges. The at least two large hole shaped charges may be arranged to generate jets oriented substantially along respective axes that converge towards one another. Thus a group of large hole shaped charges may be a convergent group, preferably oriented along the same axis as a corresponding group of deep penetrating charges. The first and/or second gun modules may each comprise a plurality of groups of shaped charges disposed on the gun module, wherein at least two groups of shaped charges overlap one another in a longitudinal direction of the gun module. At least two shaped charges within each group may be arranged to generate jets oriented substantially along respective axes that converge towards one another. The at least two groups of shaped charges may therefore be intermeshed or interlaced in a longitudinal direction of the gun module. The assembly may comprise at least two groups of shaped charges rotationally offset or phased around the longitudinal axis of the gun module. One or more groups of shaped charges may be arranged in a line parallel to the longitudinal axis of the gun module. Preferably each group of shaped charges is arranged in a line parallel to the longitudinal axis of the gun module. In one example, the assembly comprises first and second groups of shaped charges to form first and second perforation channels which overlap one another and are rotationally offset by 180 degrees. In this example, the first and second perforation channels may have first and second major diameters oriented substantially in the same plane. In one example, the assembly comprises first and second groups of shaped charges to form first and second perforation channels which overlap one another and are rotationally offset (or phased) by 60 degrees. This example may have particular benefits in applications to the perforation and fracture of high angle or horizontal wells. In such an application, the first and second perforation channels may be arranged to be substantially vertically facing or may face the top of the wellbore (i.e. a plane at the mid-point between the first and second channels may be substantially vertically oriented or oriented towards the top of the wellbore). The use of convergent shaped charges which overlap one another has benefits in hydraulic fracturing applications. The perforation channels formed by the inventive assembly are more closely aligned which aids fracture growth and placement control, as fracture initiation points are positioned axially close to one another. An additional unexpected benefit is improved gun survivability as a result of the overlapping, and optional phasing, of groups of convergent shaped charges. The intermeshing of charges promotes the distribution and dissipation of shock wave forces through the perforating gun assembly, and increases the integrity of the perforating gun assembly and tool string without compromises regarding charge placement, charge capacity, or material and manufacturing costs. Preferably the perforating gun assembly also comprises a housing, and the charge carrier is disposed within the housing. The housing may comprise a substantially cylindrical member, which may define a cylindrical bore in which the charge carrier is disposed. The charge carrier may be co-axial with the housing. In the context of this description, the term "housing" is used to refer to an outer housing or casing of the perforating gun assembly, and the term "charge carrier" is used to describe a structure to which the shaped charges are mounted (directly or indirectly) in their desired groupings and/or orientations. The perforating gun assembly may comprise an anchor for securing the assembly in a cased or lined wellbore. The assembly may comprise a positioning and alignment module configured to translate the perforating gun assembly in the wellbore. The assembly may comprise a guide shaft and a guide housing, the guide shaft being movable within the guide housing to translate the perforating gun assembly in the wellbore. The guide shaft may be movable from a first position, in which the first gun module is adjacent the casing or liner at a perforation location; and a second position in which the second gun module is adjacent the casing or liner at the perforation location such that the position of the deep penetrating shaped charges corresponds to the position of the one or more large holes. Preferably, the positioning and alignment module comprises one or more guide keys co- operating with one or more guide keyways. Preferably, the positioning and alignment module comprises one or more shear pins, which may be configured to maintain the guide shaft in its first position. The shear pins may be configured to be sheared by a force applied to the assembly. The method may comprise assisting the translation of the perforating gun assembly using a spring. The method may comprise translating the perforating gun assembly by applying a force to at least a portion of the perforating gun assembly. The force may be a motive force from a downhole tractor, which may form a part of the perforating gun assembly. Alternatively, or in addition, the motive force may be a tensile force or a compressive force or downweight applied from surface. The assembly may comprise at least one sealed chamber, and the method may comprise breaking or rupturing a seal to the sealed chamber. The sealed chamber, which may be pressure isolated from the wellbore, may comprise a liquid filled chamber in a first condition. The liquid filled chamber may prevent translation of the perforating gun assembly with respect to the anchor in its first condition. The method may comprise breaking or rupturing a seal to the sealed chamber to enable liquid to be expelled from the chamber. The method may comprise expelling liquid from the chamber during translation of the perforating gun assembly with respect to the anchor. The method may comprise breaking or rupturing a seal by detonation of the first gun module. The seal may comprise a rupture disc. The method may comprise translating the perforating gun assembly by applying a force to at least a portion of the perforating gun assembly, wherein the force is applied by exposing at least a portion of the gun assembly to a wellbore pressure. The perforating gun assembly may comprise a pressure chamber, which in its first condition is sealed and isolated from a wellbore pressure, and is at a pressure lower than a wellbore pressure. The pressure chamber may be filled with air at atmospheric pressure. The method may comprise exposing the pressure chamber to wellbore pressure, thereby causing wellbore fluids to enter the chamber. The method may comprise exposing at least a portion of the positioning and alignment module to wellbore pressure, to actuate movement of the guide shaft in the guide housing, and thereby translate the perforating gun assembly. The method may comprise exposing at least a portion of the positioning and alignment module to wellbore pressure by detonation of the first gun module. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided a method of perforating a cased or lined wellbore in a hydraulic fracturing operation, the method comprising:

- providing a first gun module and a second gun module, wherein the first gun

module comprises at least one large hole shaped charge, and wherein the second gun module comprises at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another;

- locating the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location;

- detonating the at least one large hole shaped charge of the first gun module to form one or more large holes in the casing or liner;

- detonating the deep penetrating shaped charges of the second gun module to generate jets which pass through the one or more large holes and form at least two deep penetration holes in the formation adjacent the perforation location, wherein the least two deep penetration holes intersect to provide a connected channel in the formation. Embodiments of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided a method of perforating and fracturing a cased or lined wellbore, the method comprising: - providing a first gun module and a second gun module, wherein the first gun module comprises at least one large hole shaped charge, and wherein the second gun module comprises at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another;

- locating the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location;

- detonating the at least one large hole shaped charge of the first gun module to form one or more large holes in the casing or liner;

- detonating the deep penetrating shaped charges of the second gun module to generate jets which pass through the one or more large holes and form at least two deep penetration holes in the formation adjacent the perforation location, wherein the least two deep penetration holes intersect to provide a connected channel in the formation;

- pumping a fracturing fluid into the wellbore to the perforation location and into the connected channel; and

- initiating and propagating a fracture from the connected channel. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. According to a further aspect of the invention, there is provided a perforating gun assembly substantially as described herein with reference to Figures 1 A, 1 B, 2, 3A, and 3B of the drawings. According to a further aspect of the invention, there is provided a method of performing a hydraulic fracturing operation substantially as described herein with reference to Figures 1 to 4 of the drawings. According to a further aspect of the invention, there is provided a method of performing a hydraulic fracturing operation substantially as described herein with reference to Figures 5A and 5B of the drawings. According to a further aspect of the invention, there is provided a perforating gun assembly substantially as described herein with reference to Figures 6A and 6B of the drawings. Brief description of the drawings There will now be described, by way of example only, a various embodiments of the invention with reference to the drawings, of which: Figure 1A is a schematic sectional view of a perforating gun assembly in accordance with a first embodiment of the invention in an initial detonation position; Figure 1 B is a schematic sectional view of the perforating gun assembly of Figure 1 A in a second detonation position; Figure 2 is a part-sectional view of first, lower gun module and second upper gun module of the gun assembly of Figures 1 A and 1 B; Figure 3A is a schematic sectional view of the positioning and alignment module of the gun assembly of Figure 1A, showing internal components; Figure 3B is a schematic sectional view of an alignment mechanism of the positioning and alignment module of the gun assembly of Figure 2A; Figure 4 is a schematic representation of a wellbore traversing a subterranean formation, with perforating channels formed according to an embodiment of the invention; Figures 5A and 5B are schematic representations of the result of a hydraulic fracturing operation according to an alternative embodiment of the invention applied to a horizontal well 120, from a side perspective view and a plan view respectively; Figure 6A is a schematic sectional view of a positioning and alignment module of a gun assembly of according to an alternative embodiment of the invention, showing internal components; and Figure 6B is a schematic sectional view of an alignment mechanism of the positioning and alignment module of the gun assembly of Figure 6A. Detailed description of preferred embodiments Referring firstly to Figures 1A and 1 B, there is shown schematically a perforating gun assembly, generally depicted at 10, located in a cased wellbore 11. Figure 1A shows the assembly 10 in an initial detonation position, and Figure 1 B shows the perforating gun assembly of Figure 1A in a second detonation position, as will be described below. It is an aim of the invention to provide a perforating gun assembly for hydraulic fracturing applications, and the following embodiments of the invention will be described in that context. However, the invention has application to alternative reservoir treatment operations, and its benefits may also arise where the invention is applied to injection of other fluids, such as in water and or gas flood operations, in which breakdown (fracturing) of the formation is often (but not necessarily) a consequence of such operations. The assembly 10 is designed to be deployed on a wireline or other flexible conveyance (not shown) with the assistance of a wireline tractor 12 located at the upper end 13 of the assembly. A lower packer 14 is provided at the lower end 15 of the assembly 12. A packer setting tool 17 connects the packer 14 to the assembly, enabling the packer 14 to anchor the assembly 10 against the casing 18 at its required depth and provide a fluid barrier in the wellbore at the lower end of the assembly. The assembly comprises a first lower gun module 20, axially separated from the lower end 15 of the assembly, and a second, upper gun module 30, located between the lower gun module 20 and the upper end 13 of the assembly. A first detonator module 21 is associated with the lower gun module 20, and a second detonator 31 is associated with the upper gun module 30. The first and second detonation modules 21 , 31 are controlled from surface and are independently switchable to detonate the first and second gun modules respectively. A centraliser device 40 provides centralisation and support for the assembly, and in this example is located between the upper gun module 30 and the wireline tractor 12. A positioning and alignment module 50 connects the packer 14 to the lower gun module 20 via the packer setting tool 17. The positioning and alignment module 50, which will be described in more detail below, functions to enable translation of the assembly between defined first and second detonation positions shown in Figures 1A and 1 B respectively. A part-sectional view of the lower and upper gun modules 20, 30 is shown in Figure 2. Each of the lower and upper gun modules 20, 30 comprises a housing 22, 32 which forms a casing for the internal components of the gun module. Each housing is a substantially cylindrical hollow tube, with an internal bore sized to receive and accommodate a charge carrier. The charge carrier functions to support a number of shaped charges 24a-f, 34a-f. The shaped charges are ballistic elements as are known in the field of wellbore perforation and hydraulic fracturing. Each includes a housing, a liner, and a quantity of explosives between the housing and the liner. On detonation the shaped charges generate a high pressure stream of particles referred to as a jet. The jet direction depends on the orientation of the shaped charge within the gun assembly. The charge carriers of this embodiment are substantially cylindrical hollow tubes of unitary construction which extend over an axial length of the gun assembly. Arranged over its length are a number of apertures, which extend from the outer wall of the charge carrier to a bore defined by the charge carrier. Each aperture is sized and shaped to receive a shaped charge in its desired orientation. Corresponding mounting holes are located diametrically opposite the apertures and provide a fixing point for the shaped charge in its desired orientation. The apertures and holes enable the shaped charge to be mounted at a fixed angle with respect to a plane perpendicular to the longitudinal axis L of the charge carrier. The orientation of the apertures and the mounting holes together determines the orientation of the shaped charges, and consequently the direction of jets resulting from the detonation of the shaped charges and the nature of the perforations formed. The shaped charges 24 of lower gun module 20 are large hole (or big hole) charges, each of which is configured to form a relatively large diameter hole in the casing 18 and cement lining 19. A typical large hole charge might comprise around 22.7 grams of explosives, and be designed to create an entry hole having a diameter of approximately 0.7 inch to 1 inch (around 18mm to 25mm), with a normalised penetration depth of the order of 6 to 9 inches (150mm to 225mm). In this embodiment, the large hole charges are designed to create a hole in the casing and cement lining having a diameter of approximately 20 to 25mm (0.75 inch to 1 inch). The shaped charges 34 of the upper gun module 30 are high energy deep penetrating charges, each of which is configured to form a relatively small diameter channel which penetrates into the formation. A deep penetrating charge might comprise around 22.7 grams of explosives, and be designed to have a normalised penetration depth (in a cased hole) of the order of 20 to 27 inches (500mm to 700mm) with an entry hole having a diameter of approximately 0.25 inch to 0.45 inch (around 6mm to 11 mm). In this embodiment, the deep penetrating hole charges are designed to be capable of forming a hole in a casing and cement lining with a penetration depth of around 600mm. It will be appreciated that charges of different weights may be used in the first and/or second gun modules, and the invention is not restricted to configurations which have equal charge weights in the first and second sets of charges. The shaped charges of each gun module 20, 30 are arranged in functional groups of converging charges. In upper module 30, a first group of shaped charges 35 comprises deep penetrating shaped charges 34a, 34c and 34e, all of which are oriented in a line parallel to the longitudinal axis such that they are in the same plane parallel to the longitudinal axis. Two of the charges, 34a and 34e, are oriented with their axes at angles inclined to a plane which is perpendicular to the longitudinal axis of the charge carrier such that they converge towards one another. In addition, the axis of charge 34c is oriented towards the point of intersection of the axes of charges 34a and 34e. The three shaped charges in the group are therefore oriented to converge to the same point in the formation. A second group of shaped charges 36 is formed from deep penetrating charges 34b, 34d, and 34f, all of which are oriented in a line parallel to the longitudinal axis such that they are in a plane parallel to the longitudinal axis. The plane of the second group is rotationally offset or phased with respect to the first group. In this case the phasing angle θι is 180 degrees, so that the first and second groups are oriented in the same plane but in opposing directions. Charges 34b and 34f are oriented with their axes at angles inclined to a plane which is perpendicular to the longitudinal axis of the charge carrier such that they converge towards one another. In addition, the axis of charge 34d is oriented towards the point of intersection of the axes of charges 34a and 34e. The three shaped charges in the second group are therefore oriented to converge to the same point in the formation. The first and second groups of charges are intermeshed or interlaced such that they overlap over the axial direction of the charge carrier; each group extends over an axial portion which overlaps a perpendicular plane. This configuration of converging charge groups has certain advantages in fracturing applications, as will be described below. In the first (lower) gun module 20, the large hole shaped charges 24 are also arranged in functional groups of converging charges, with orientation axes corresponding to the axes of the deep penetrating charges 34. A first group of shaped charges 25 consists of shaped charges 24a, 24c and 24e, and a second group of shaped charges 26 is formed from 24b, 24d, and 24f. The first and second groups of charges of the lower gun module are also intermeshed or interlaced such that they overlap over the axial direction of the charge carrier. The perforating gun assembly 10 also contains a detonation system which in this case comprises a pair of detonation modules 21 , 31 and corresponding detonation cords (not shown) which run along the length of the bore of the charge carrier, connecting the shaped charges of their respective modules in series. Each detonation module 21 , 31 is independently controllable from surface to detonate the respective gun modules. Other detonation arrangements, such as those that use different detonation sequences or timing delays, may be used in alternative embodiments and are within the scope of the invention. Figures 3A and 3B show schematically details of the positioning and alignment module 50 of the assembly 10, suitable for use with an assembly which is deployed using a motive force, such as a wireline tractor, coiled tubing, or drill pipe. Figure 3A shows the tool 50 in longitudinal section, and Figure 3B is a cross section through line B-B ¹ . The tool 50 is located between the lower gun module 20 and the packer setting tool 17, and is shown in Figure 3A in a closed position. The tool comprises a guide shaft 51 extending from the lower gun module 20 into a tubular guide body 52 connected to the lower end of the assembly. An upper end 53 of the guide body 52 has an end plate 54, with an aperture 55 for receiving the guide shaft 51. A sealing ring 56 seals the end plate 54 against the guide shaft 51. The guide body 52 defines a closed chamber 58 which is filled with a fluid. A bore 59 extending through the guide shaft 51 provides fluid communication to the lower gun module 20, which prior to operation, is sealed by a ported rupture disc sub assembly 60. The bore 59 also accommodates an electrical control line 62 which passes through the chamber 58 to connect the packer setting tool 17 to surface. At a lower end 64 of the guide shaft 51 is a pair of guide keys 66. The guide keys 66 extend radially outward of the guide shaft 51 into keyways 68 of the guide body 52. The guide shaft is therefore able to slide within the guide body in use, but is rotationally keyed with respect to the guide body. The guide keys 66 are initially secured in the keyways by shear pins 70, preventing movement of the assembly prior to actuation. A power spring 72 is disposed between the end plate 54 and a shoulder located adjacent the guide keys. A method of use of an embodiment of the invention will now be described with reference to Figures 1A, 1 B, 2, 3A and 3B. Referring firstly to Figures 1 A and 3A, the assembly 10 is deployed on an electric wireline or other flexible conveyance to its required depth in the cased wellbore. When located in the required position, the packer setting tool 17 is actuated via the electrical control line 62 to set the packer 14 and anchor the assembly against the casing 18. With the packer set, the lower gun module 20 is ready to be detonated. The shear pins 70 are intact and retain the positioning and alignment module in its spaced-out position, as shown in Figure 1A. In addition, the fluid chamber 58 remains closed and pressure isolated. The chamber 58 and bore 59 define a fixed fluid volume to the rupture disc sub assembly 60, preventing downward movement of the guide shaft 51 and other components of the assembly. The large hole charges of the gun module 20 are detonated, resulting in the formation of discrete casing entry holes 41 which penetrate the casing 18 and cement 19 to connect the wellbore with the formation immediately outside of the wellbore. The objective of the first stage detonation is to create large casing entry holes for improved hydraulic fracture performance. Penetration into the formation is of secondary performance, and the shaped charges are designed to deliver minimal connection to the reservoir and formation while maximising entry hole geometry. The charges 24 may therefore be optimised to provide large diameter casing entry holes 41. During detonation of the first gun module 20, the pressure and forces generated from the explosion event are sufficient to break the rupture disc in the sub assembly 60, enabling pressure communication between ports (not shown) in the sub assembly 60 and the chamber 58. The wireline tractor 12 is engaged to provide a downward motive force on the assembly 10, which is transferred through the guide shaft 51 to the guide keys 66 to shear the pins 70. The guide shaft 51 is released from the guide body 52 and the downward motive force from the wireline tractor 12 causes the shaft 51 to move downwards in the guide body. Movement is assisted by the power spring 72. As the shaft 51 moves downwards, fluid from the chamber 58 exits through the bore 59 and the ports (not shown) in the rupture disc sub assembly 60. The shaft 51 is only permitted to move with respect to the guide body and packer by a fixed predetermined distance, which is selected to position the shaped charges of the second (upper) gun module 30 directly over the casing entry holes 41 formed by the first detonation, as shown in Figure 1 B. A consequence of the assembly being anchored to the packer 14 at the lower end of the assembly is that the operation is executed from a fixed depth reference. Throughout travel of the assembly to its lower position, rotation of the assembly is prevented by the arrangement of guide keys and keyways. With the assembly 10 in the second, lower position as shown in Figure 1 B, the second gun module 30 is ready to be detonated. Optionally, a latch mechanism (not shown) is provided to lock the guide shaft into its lower position, so that when the assembly is recovered to surface, it can be inspected to ensure that the components were properly translated to their required positions before the second detonation. Detonation module 31 is fired to detonate the deep penetrating charges 34 of the second module. The charges are oriented such that they create jets which pass through the casing entry holes 41 and into the formation to penetrate the formation beyond the near-wellbore damaged formation, and form interconnected cavities 42 adjacent the wellbore. The objective of the second stage detonation is to create deep penetrating holes through the pre-existing casing entry holes, which interconnect in the formation. The hole diameter created in the second stage detonation is of secondary performance, since hydraulic performance has already been optimised by the creation of large entry holes in the first stage detonation. The design of the charges 34 may therefore be optimised to provide deep penetrating holes in the formation. The penetration depth of the charges 34 can be expected to be significantly greater than the typical normalised penetration depth achieved by the same shaped charge design operating in a single event perforation operation in a cased hole. Firstly, the first stage operation has already formed a connection with the formation and therefore the jets created by the charges of the second gun are not impeded by the casing or cement. Secondly, the first stage detonation will have weakened the rock in the perforation location, improving the penetration of the second gun module jets into the formation. Weakening of the rock may be enhanced by orienting the charges of the first gun module along the same convergent axes as the deep penetrating charges of the second gun module. Optionally, the packer 17 is left in place in the wellbore to act as a zonal isolation device, which separates the treatment zone from an adjacent zone during a subsequent fracturing treatment. Figure 4 shows schematically the wellbore 11 and formation 100 subsequent to detonation of the upper gun module 30. In the formation, the individual jets created by the high energy deep penetrating charges 34 extend through the casing entry holes formed by the converge towards one another and interact to form a pair of interconnected perforation channels 42a, 42b. The perforation channels have their major diameters substantially parallel to the longitudinal axis of the wellbore 11 , and overlap in a longitudinal direction of the wellbore 1 1. The channels are substantially triangular in section, with their apexes 44a, 44b in the formation beyond the near-wellbore damage zone. The apexes 44 of the channels 42a, 42b each create single fracture initiation site. During a hydraulic fracturing operation, hydraulic fluid is pumped from surface down to the perforation location, and exits the large casing entry holes. The large flow area created by the holes mitigates perforation friction pressure losses, and therefore reduces the pumping pressure needed to generate the desired fracture initiation and propagation flow rates. This reduces the necessary pumping capacity required at surface. The large hole area also reduces the risk of early screen out of sand, which may bridge smaller entry holes, and reduces the axial extent over which the entry holes need to be formed, shortening the perforation interval and enhancing fracture placement. Hydraulic fluid pumped into the channels 42a, 42b is directed towards the apex 44a, 44b of the channel. The convergence of flow from multiple entry holes and towards the apex provides a single fracture initiation site which enables single planar fracture growth. In contrast with previous proposals, the multiple entry holes of the described embodiment do not risk create competing or multiple fracture initiation sites: flow from each entry hole feeds a single fracture initiation point due to the hydraulic connection of the channels in the formation outside of the casing. The fracturing operations of the described embodiment benefit from a reduction in breakdown pressure and a reduction in treatment pressure. The invention enables lower usage of chemicals and materials and may also benefit from increased fracture conductivity. Advantageously, the single fracture initiation site is located beyond the damaged zone and hoop stress affected region of the formation. By utilising a two-stage detonation process, it is possible to use convergent shaped charge groups oriented to intersect at a point relatively deep into the formation. The described method and apparatus assists in achieving a Limited Entry fracturing effect. Limited Entry fracturing is desirable as, by design, each perforation is in hydraulic communication and contributes fluid during the fracturing treatment, and therefore offers improved control over the initiation and growth of a fracture from within a given perforation network. In longer perforation intervals, fracture tortuosity and multiple fracture events, both of which hamper the design objective of Limited Entry, can be prevalent. The issues are exacerbated in high angle or horizontal wells where regional (tectonic) stress effects dominate the fracture growth pattern, with the perforation itself being a secondary factor. Limiting the perforation interval to a very short section (for example as low as 1ft to 2ft or 0.3m to 0.6m) can have benefits towards reducing the fracture tortuosity and multiple fracture events, and facilitating Limited Entry fracturing, particularly in high angle or horizontal wells. In the described embodiment, the pair of perforation channels 42a, 42b are closely aligned due to the intermeshing of the charge groups. This aids fracture growth and placement control, as fracture initiation points are closely aligned and at 180 degrees to one another (i.e. arranged in the same plane). Figures 5A and 5B show schematically the result of a hydraulic fracturing operation according to an embodiment of the invention, applied to a horizontal well 120. Figure 5A is a side perspective view of the wellbore and perforation geometry, and Figure 5B is a plan view of the wellbore and perforation geometry. A perforating gun assembly 130, which is similar to the perforating gun assembly 10 of Figures 1 to 3, comprises first and second gun modules. The first gun module is configured to detonate two groups of three large- hole shaped charges to form large casing entry holes in a first stage detonation. The second gun module is configured to detonate two groups of three convergent deep- penetrating shaped charges to form deep penetrating holes through the casing entry holes in a second stage detonation. In this embodiment, the first and second groups of shaped charges are oriented to form connected perforation channels 121a, 121 b which are phased at 60 degrees to one another, and are oriented generally in a vertically upward direction. In this embodiment, the preferred fracture direction is the direction of maximum horizontal stress 124, and the wellbore is drilled predominantly in the direction of minimum horizontal stress 125. The apexes 122a, 122b of the channels 121 a, 121 b are the convergent points for delivery of fracturing fluid, and are aligned in a single plane 126 which is substantially aligned with the preferred fracture direction (the direction of maximum horizontal stress 124). This arrangement promotes fracture initiation from the wellbore 120 which cross cuts both perforation channels 121a, 121 b in the preferred orientation. The perforation interval has been limited to a very short section, which reduces a tendency for fracture tortuosity and multiple fracture events, and facilitates Limited Entry. It will be appreciated that in other embodiments of the invention, different configurations of convergent charge groups, collections and phasing angles may be used. For example, a collection may comprise N groups of shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another and create perforations which interact to form N perforation channels. The number of collections of shaped charges in an assembly may be varied according to the application, and is not limited to the embodiments described herein. Phasing angles may be selected according to the application, well geometry, or formation conditions. It will also be appreciated that in alternative embodiments of the invention the perforation channels may overlap to a lesser extent, or be axially separated along the wellbore. Multiple gun modules can be run on a single perforating gun assembly, so that the initial large hole detonation operation and second deep penetrating detonation operation are carried out simultaneously at multiple locations along the wellbore. This enables multiple perforation channels to be formed at separate wellbore intervals on a single run. Alternatively (or in addition), the principles of the invention may be extended to incorporate third, fourth, and/or further gun modules for sequential detonation of three or more gun modules at specific perforation locations. Each of the detonation stages may be aligned with the previous stage to create successively deeper penetrations through the initially formed large casing entry holes. The above-described embodiment of the invention comprises a positioning and alignment module 50 suitable for use with an assembly which is deployed using a motive force, such as a wireline tractor, coiled tubing, or drill pipe. However, the principles of the invention are also applicable where there is no motive force available, and Figures 6A and 6B are schematic views of a positioning and alignment module 150 of a perforating gun assembly according to an alternative embodiment of the invention. Figure 6A shows the module 150 in longitudinal section, and Figure 6B is a cross section through line B-B ¹ . The module, generally depicted at 150, is similar to module 50 of Figures 1 A, 1 B, 3A, and 3B and its principles of operation will be understood from those Figures and their accompanying description. However, the assembly 150 differs in several respects which will be described below. The module 150 is located between the lower gun module 20 and the packer setting tool 17, and is shown in Figure 6A in a closed position. The tool comprises a spacer shaft 151 extending from the lower gun module 20 and connecting to a tubular inner piston mandrel 152 connected to the lower end of the assembly. The inner piston mandrel 152 extends into an outer piston mandrel 157. An upper end 153 of the outer piston mandrel 157 has an opening 155 for receiving the inner piston mandrel 152. A sealing ring 156 seals the outer mandrel against the inner mandrel. The outer piston mandrel 157 defines a closed chamber 158 which is fluidly connected with the internal volume of the lower gun module 20 via a ported sub assembly 160, and is filled with air at atmospheric pressure and isolated from well pressure. A pathway extending through the module provides a pathway for an electrical control line 162 which passes through the chamber 158 to connect the packer setting tool 17 to surface. At a lower end 164 of the inner piston mandrel 152 is a pair of guide keys 166 in the form of lugs extending radially outward of the inner piston mandrel 152 into keyways 168 provided in the outer piston mandrel 157. The inner piston mandrel 152 is therefore able to slide within the outer piston mandrel 157 in use, but is rotationally keyed with respect to the outer piston mandrel 157. The mandrels 152, 157 are initially secured to one another by shear pins 170, preventing movement of the assembly prior to actuation. The outer piston mandrel 157 comprises reduced inner diameter portions 174, 176 located at the upper end 153 and an axially separated position towards the lower end. The reduced inner diameter portions define an annular piston chamber 177 between the inner and outer piston mandrels. A piston flange 178 upstands from the inner piston mandrel 152 and carries an o-ring seal 180 to seal with the inner surface of the outer piston mandrel 157. A damping spring 172 is disposed between the piston flange 178 and a shoulder defined by the reduced inner diameter portion 176. An o-ring seal 182 seals the inner piston mandrel 152 against the reduced inner diameter portion 176. Pressure entry ports 181 connect the inside of the inner piston mandrel 151 with the piston chamber 177. In use, a perforating gun assembly comprising the positioning and alignment module 150 will be operated in a similar manner to the assembly 10, and will be understood from the description of Figures 1A, 1 B, 2, 3A, and 3B. The assembly is deployed on an electric wireline or other flexible conveyance to its required depth in the cased wellbore. When located in the required position, the packer setting tool 17 is actuated via the electrical control line 162 to set the packer 14 and anchor the assembly against the casing 18. With the packer set, the lower gun module 20 is ready to be detonated. The shear pins 170 are intact and retain the positioning and alignment module in its spaced-out position, as shown in Figure 1A. In addition, the chamber 158 remains closed and pressure isolated from the wellbore. The large hole charges of the gun module 20 are detonated, resulting in the formation of casing entry holes 41 which penetrate the casing 18 and cement to connect the wellbore with the formation immediately outside of the wellbore. After detonation of the first gun module 20, the internal volume of the gun module is exposed to well pressure, and wellbore fluids enter the gun module and pass through the ported sub assembly 160 to the inside of the inner piston mandrel 152. Well fluids flow under well pressure through the pressure ports 181 and into the piston chamber 177, and flow into the chamber until the volume is filled. The force from the well fluids acts on the relatively large face of the piston flange 178 and exceeds the force on the relatively small lower end 183 of the inner piston mandrel 152. The net force overcomes the rating of the shear pins 170 and pushes the inner piston mandrel 152 and upper assembly downwards in the outer piston mandrel 157. Movement is damped by the spring 172. As with the previous embodiment, the upper assembly is only permitted to move with respect to the outer piston mandrel 157 and packer 14 by a fixed predetermined distance, which is selected to position the shaped charges of the second (upper) gun module 30 directly over the casing entry holes 41 formed by the first detonation, in the manner shown in Figure 1 B. With the assembly 10 in the second, lower position as shown in Figure 1 B, the second gun module 30 is ready to be detonated. Further alternatives to the described embodiments are envisaged within the scope of the invention. The above-described embodiments of the invention comprise a charge carrier formed from a substantially cylindrical hollow tube of unitary construction. However, alternative embodiments of the invention may comprise a charge carrier of a different form. For example, the charge carrier may comprise a solid substantially cylindrical mandrel with machined recesses. In variations to the described embodiments, the large hole shaped charges of the first gun module are not designed and oriented to create a number of discrete holes, but are instead arranged to create a large aperture or slot in the casing or lining which connects the wellbore to the formation. Although the embodiments described include groups of three large hole charges in the first gun module, alternative embodiments may comprise a greater or lesser number of large hole charges to form the casing entry holes. In particular, it may be advantageous in some configurations to use fewer large hole shaped changes (even a single large hole charge) to create a casing entry hole or slot through which the shaped charges of the second gun module are oriented. The embodiments described include a combined anchor and packer, but alternative embodiments may comprise an anchor only, which provides a fixed point of reference to execute the perforation and which can be recovered after completion of the zonal perforation exercise, but which does not provide a fluid barrier or seal. Further alternatives may comprise a packer and anchor system, which is operated to set the packer at a first downhole location, where it is released to be left in hole. The assembly may then be moved back up-hole to the intended perforating location where the anchor is set to provide a fixed point of reference to execute the perforation stages. Once complete, the gun is recovered along with the anchor. The benefit of this system is that the anchor can be designed to absorb the shock load from the gun. The packer, having been set some distance away from the intended perforation interval, is not affected by shock waves from the detonation of the gun modules. This enables a standard packer to be used, rather than one which is specially engineered to cope with the detonation shocks. The invention provides a method and apparatus for perforating a cased or lined wellbore in a hydraulic fracturing operation. The apparatus comprises a perforating gun assembly comprising a first gun module on a first longitudinal portion of the assembly and a second gun module on a second longitudinal portion of the assembly. The first gun module comprises at least one large hole shaped charge, and the second gun module comprises at least two deep penetrating shaped charges arranged to generate jets oriented substantially along respective axes that converge towards one another. The method comprises locating the perforating gun assembly in a cased or lined wellbore with the first gun module adjacent a casing or liner at a perforation location and detonating the large hole shaped charges of the first gun module to form one or more large holes in the casing or liner. The perforating gun assembly is translated in the wellbore to locate the second gun module adjacent the casing or liner at the perforation location such that the position of the deep penetrating shaped charges corresponds to the position of the one or more large holes; and the deep penetrating shaped charges of the second gun module are detonated to generate jets which pass through the one or more large holes and form at least two deep penetration holes in the formation adjacent the perforation location. The least two deep penetration holes intersect to provide a connected channel in the formation. The invention provides a perforating gun assembly and a method of use in hydraulic fracturing applications which addresses drawbacks and deficiencies of previously proposed apparatus and methods. In particular the invention provides a multiple stage perforating gun assembly and a method of use in hydraulic fracturing applications which provides improved perforation connectivity and fracture initiation, while mitigating a requirement for unnecessarily high pumping pressures during fracturing. Embodiments of the invention provide discrete fracture initiation sites with improved placement control and/or improved hydraulic efficiency. A principle benefit of the invention is that compromises between hydraulic efficiency and control may be avoided by providing both large diameter casing entry holes and interconnected deep penetrating holes in a multistage operation using the same gun assembly. Various modifications to the above-described embodiments may be made within the scope of the invention, as defined by the appended claims.