This invention relates to the determination of wellbore stab iiity and particularly the influx or efflux of fluids to or from a borehole during a drilling operation.

When drilling a borehole for a well, such as an oil or gas well, it is desirable to be informed when fluid is passing through the walls of the borehole between the borehole itself and the formation through which the borehole is passing. Whether a formation fluid, such as water, oil or gas is leaking/flowing out of the formation into the borehole, or drilling fluid (mud) within the borehole is being lost into the formation, it is necessary to know this in order to continue the drilling process safely and efficiently.

It has now been discovered that in certain circumstances useful information about the influx or efflux of fluid can be gained by observing the pressure within the borehole in the region of the drilling bit, this observation being carried out not when the drill is actually operating but when it is still (and the pipe string to which it is mounted is also still) and the drilling fluid circulation is stopped. The basis for this new approach to determining wellbore stability is that the drilling fluid is thixotropic (like a non-drip paint), and when allowed to • when not disturbed by motion - will form a gel which can be advantageously used as a medium of measurement, as is now explained.

Rather surprisingly it has been found through experimental work that a gelled drilling fluid is capable of accurately transmitting the force created by moving fluids at the interface between the borehole and the earth formation being drilled through - that is inflowing or outflowing fluids - displacing the drilling fluid along the borehole, and with an efficiency far greater than previously recognized. As long as the drilling fluid is effectively gelled, it acts like a solid in transmitting pressure, and can therefore respond to and transmit pressure changes due to volume changes occurring near the drill bit with great sensitivity, even if the relevant pressure sensors are themselves located some distance away. Of course, to use this capability the drilling fluid must actually be in a gelled state, and that means that the drill bit and drill string must not be in motion, and the drilling fluid must not be in active circulation, i.e. not being pumped from surface, along the borehole. Thus the present invention comprises a method and apparatus for determining fluid inflow or outflow during drilling, by using a gelling

drilling fluid whose characteristics - yield stress τ _y and gelation period t , - are known, and then, while drilling motion and pumping are stopped, measuring downhole differential pressure ΔP and using the observed changes therein to allow a determination of the fluid flow.

In one aspect, therefore, the invention provides a method of determining fluid movement into or out of a borehole from or to the surrounding formation during drilling of the borehole using a gelling drilling fluid, the method including the steps of:

• determining the yield stress τ _y and gelation period t _g of the drilling fluid;

• stopping drilling, rotation and pumping, and, while keeping the drilling string stationary for a period of time t _g , measuring the downhole differential pressure ΔP; and

• from the observed changes in differential pressure ΔP during the period t _g , determining the fluid flow.

In a second aspect the invention provides apparatus for use in the method of the invention, which apparatus, comprises:

• a bottom hole assembly for drilling a borehole;

• a differential pressure monitor, affixed to the bottom hole assembly and operative to measure the differential pressure of fluid in the borehole along the longitudinal axial orientation of the borehole; and

• means for communicating the output of the differential pressure monitor to the surface.

The preferred forms of both the method and the apparatus of the invention will be seen from the following description.

The preferred form of apparatus employs a differential pressure monitor; this is conveniently two individual pressure sensors located on the exterior of the bottom hole assembly and suitably spaced apart from each other along the axial orientation of the

borehole (preferably by a distance greater than one foot [about 30cm]). The pressure monitor is desirably positioned near the bottom end of the bottom hole assembly.

Where individual pressure sensors are used in the pressure monitor, they advantageously each comprise a quartz pressure sensor having a resolution of at least 0.01 psi (60 Pa) and a range of on the order of 20 thousand psi (130 MPa) (conveniently from 0 to 20k). Alternative pressure sensors are well known in the industry.and may be used advantageously by those skilled in the art.

The data gathered by the pressure monitor is best recorded - stored - for subsequent use in whatever determination calculations are to be carried out, and rather than transmit the data directly up the string and to some suitable ground/surface equipment, preferably it is stored within the bottom hole assembly. It may then either be utilised there (by appropriate calculating means), or sent up to the surface.

Once gathered, and stored, the pressure monitor's data can be input to means for determining the borehole fluid influx or efflux, responsive to the output of the differential pressure monitor. This means includes means for determining the change in the measured differential pressure of the gelled drilling fluid in the borehole over a period of time at least equal to the gelling time t _g of the fluid.

The determination of the relevant fluid flow involves a number of factors. Firstly, it requires a knowledge of the drilling fluid characteristics (which may be measured, either in advance or during the drilling process) to allow a determination of the yield stress τ _y over the gelation time t _g of the fluid. A Fann rheomoter may be used for this purpose. The measured values of τ _y as a function of time are compared to an equation of the form

A = τ _y /t" (1 )

(where A and n are constants, and is time) using a fitting program such as one based on least square fit, to extract the values of the constants A. and n-

A period of time t _g is needed for gelation to occur, and this is typically about several seconds to several minutes depending on the type of drilling fluid used as well as on the downhole temperature and pressure conditions. During this period t , the

differential pressure downhole is measured using the pressure sensors P1 and P2. If the differential pressure ΔP is constant during the period t _y it is determined that there is no influx taking place. If, however, the differential pressure ΔP is changing then that indicates that fluid flow is occurring. An increasing differential pressure shows that an influx of formation fluids into the borehole is taking place, while a decreasing differential pressure shows that a reverse-influx (that is, an efflux) of drilling fluids into the formation is occurring. The detection of an influx condition can be utilised to trigger an alarm at the surface, prompting the driller to take any required remedial action. Such remedial actions may entail modifying the density or weight of drilling fluids to be pumped into the well, modifying the use of special additives to the drilling fluid, shutting in the well, or other steps well known in the drilling industry.

It is optionally possible to determine the influx flow rate a using the following relationship:

q = ΔP/KAt ^{ln+1 >} (2)

K - ( 96 * L) / [ ( d _c - d, ) ' . ( ά ² _ d ) ] (3)

(where d ₀ is the diameter of the borehole, and d, is the diameter of the bottom hole assembly).

As has been noted hereinbefore, in order to promote the gelation of the drilling fluid, all motion of the bottom hole assembly is stopped, by stopping drilling, stopping rotation of the drill string, and stopping pumping of the drilling fluid. Normally, this is done at every change of a stand of drill pipe, and the entire drill string is also lifted off bottom. However, the method of the present invention can be performed more frequently, and at any time that it is desired to detect whether an influx is occurring.

Embodiments of the invention are now described, though by way of illustration only, with reference to the accompanying diagrammatic Drawings in which:

Figure 1 shows a side see-through view of a bottom hole assembly incorporating the apparatus of the invention;

Figure 2 shows a representation of the sequence of events that might occur using the apparatus and method of the invention;

Figure 3 shows a Flow Diagram setting out the stages of the method of the invention; and

Figures 4-6 are graphs showing details of pressures to be seen under appropriate circumstances, and how the data can be fitted to a curve to reveal certain constants.

In the preferred form of the invention's apparatus as shown in Figure 1 a bottom hole assembly (BHA) for a drilling apparatus is provided, with a differential pressure measuring system built-in. This pressure measurement system comprises two pressure sensors P1 and P2, spaced apart along the longitudinal direction of the BHA. The pressure sensors are quartz pressure sensors having a range of 0 - 20,000 psi (130 MPa) and a resolution of 0.01 psi (60 Pa).

As can be seen from Figure 2, in operation the pressure measurement is conditioned in a signal conditioning unit, and then stored in a downhole memory 6. The signals may then be transmitted uphole using signal transmission unit 7, either immediately or - and preferably - at a later time in a delayed-transmit mode of operation. The signals are received by a surface receiver 8, passed through a decoder 9, and processed in an interpretation unit 10 and alarm unit 11.

Figures 3-6 relate to utilising the apparatus during the drilling of a borehole using a gelling drilling fluid. Figure 3 - the logic flow diagram - illustrates the steps described herein, and speaks for itself.

Figure 4 shows the measured values of drilling fluid yield stress τ _y as a function of time i, and Figure 5 shows how these are compared with Equation 1 (above) to

permit extraction of the constants A. and n-

A period of time t _g is needed for gelation to occur, and this is typically about several seconds to several minutes depending on the type of drilling fluid used as well as the downhole temperature and pressure conditions. During this period t _g , the differential pressure downhole is measured using the pressure sensors P1 and P2. If the differential pressure ΔP is constant during the period t _g , as seen in Figure 5, it is determined that there is no influx taking place. If however, the differential pressure ΔP is increasing as shown in Figure 6, then it is determined that an influx of formation fluids into the borehole is taking place. If, on the contrary, the differential pressure ΔP is decreasing as shown in Figure 7, then it is determined that a reverse-influx, or efflux, of drilling fluids into the formation is taking place. The detection of an influx condition can trigger an alarm at the surface, prompting the driller to take any required remedial action.