PRODUCTION

BACKGROUND

[0001] Following the cessation of drilling operations, completions may be initiated in which downhole tubulars and equipment are installed to enable the safe and efficient production from an oil or gas well. During completions, sections of casing or pipe string may be placed into the wellbore to enhance wall strength and minimize the chances of collapse, burst, or tensile failure. Well casings of various sizes may be used, depending upon depth, desired hole size, and types of geological formations encountered. The casing and other tubulars may, in some instances, be stabilized and bonded in position using various physical and chemical techniques.

[0002] When cement or other settable compositions are used to stabilize completion equipment, a portion of the drilling fluid may be removed from the wellbore so that the casings may be cemented in place. Primary cementing operations may fill at least a portion of the annular space between the casing and the formation wall with a hydraulic cement composition. The cement composition may then be allowed to solidify in the annular space, thereby forming an annular sheath of cement. During stimulation (such as hydraulic fracturing or other techniques), cement may provide an impermeable barrier that prevents the migration of stimulation fluids between zones to be stimulated in the wellbore. In some cases when needed, cement may provide an impermeable barrier that prevents the migration of undesired fluids and gases (e.g., water) between zones penetrated by the wellbore during. Other situations arise where cementing particular zones within a formation may be beneficial. For example, cementing operations may also include use of cement during remediation of lost circulation or zonal isolation.

SUMMARY [0003] This summary is provided to introduce a selection of concepts that are described further below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0004] In one aspect, embodiments disclosed herein relate to a method of optimizing well stimulation that includes characterizing a cement job within one or more intervals in a wellbore; determining anticipated regions of fluid communication within the cement job; and designing a stimulating treatment plan that avoids stimulating the anticipated regions of fluid communication.

[0005] In another aspect, embodiments disclosed herein relate to a method of optimizing well stimulation that includes characterizing a cement job within one or more intervals in a wellbore; determining anticipated regions of fluid communication within the cement job; and designing an engineered stimulating treatment plan for the one or more intervals in the wellbore based on the anticipated regions of fluid communication, the engineered stimulating treatment plan having a plurality of first stimulation stages that are more closely spaced within the one or more intervals than a geometric stimulation plan for the one or more intervals.

[0006] Other aspects and advantages of the disclosure will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF FIGURES

[0007] FIG. 1 is an illustration of a completions operation in which cement is installed in an annular region created between a borehole and an installed casing in accordance with embodiments of the present disclosure.

[0008] FIGS. 2 and 3 are graphic depictions illustrating a fracture operation in accordance embodiments of the present disclosure.

[0009] FIG. 4 is a schematic depicting a flow diagram in accordance with embodiments of the present disclosure. [0010] FIG. 5 is a schematic depicting a flow diagram in accordance with embodiments of the present disclosure.

[0011] FIG. 6 is a computer system in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

[0012] In one aspect, methods in accordance with this disclosure are directed to the design of completions methods that include the characterization of cementing jobs within an interval of a wellbore, and the design of subsequent fracturing operations. More specifically, one or more embodiments of the present disclosure may be directed to the design of stimulation treatments that avoid regions of fluid communication within the cementing job. Further, one or more embodiments may be directed to the design of engineered stimulation treatments, whereby the engineered stimulation treatment includes a plurality of stimulation stages that are more closely spaced than a geometric completion, such as to capitalize on regions of the cement job having high isolation quality and avoid regions of the cement job having anticipated fluid communication.

[0013] In other aspects, methods in accordance with the present disclosure may be directed to the design of cementing jobs using modeling techniques that estimate the degree of isolation of various regions of the wellbore once the cement has been emplaced and cured. Methods in accordance with the may include techniques for evaluating and determining cement hydraulic isolation, and combining the information with formation evaluation to maximize stimulation efficiency by minimizing uncertainty when selecting regions for perforation and stimulation during subsequent operational stages. In one or more embodiments, cement characterization methods may include the creation of a cement quality map for sections of a wellbore that may be used to optimize the formulation and placement of a cementing job and/or used to design stimulation operations that avoid cemented regions that have low quality or suboptimal hydraulic isolation.

[0014] In one or more embodiments, methods in accordance with the present disclosure may include performing a cementing job, characterizing the cementing job (such as by preparing a map of cement quality that may indicate regions of fluid communication in the near and far wellbore areas), and designing a stimulating treatment based on the obtained data. In some embodiments, methods may be executed using computer software that optimizes the characterization of the degree of cement isolation in one or more zones within the wellbore, and may assist users in placement of perforations and other wellbore stimulation techniques that minimize interzonal communication in the near- and far-wellbore. Computer software in accordance with the present disclosure may incorporate cement isolation information and characterization, including generating cement quality maps for a given wellbore.

[0015] Cementing operations may proceed by emplacing a cement within a wellbore, such as in an annulus created between a wall of the formation and a section of installed casing. With particular respect to FIG. 1, a derrick 100 is shown installed on a wellbore 101 traversing a formation 102. Within the wellbore 101 concentric segments of casing 104 are nested within each other, in preparation for installation of a cement sheath between the outside of the casing and the exposed formation and/or other emplaced casing strings. During the cementing operation, a cement slurry 106 is pumped into an annulus formed between formation 102 and the casing 104. In some embodiments, cement slurry may be pumped into multiple annular regions within a wellbore such as, for example, (1) between a wellbore wall and one or more casing strings of pipe extending into a wellbore, or (2) between adjacent, concentric strings of pipe extending into a wellbore, or (3) in one or more of an A- or B-annulus (or greater number of annuli where present) created between one or more inner strings of pipe extending into a wellbore, which may be running in parallel or nominally in parallel with each other and may or may not be concentric or nominally concentric with the outer casing string.

016] During wellbore stimulation, a wellbore may be perforated in a number of different locations in order to increase production, either in the same hydrocarbon-bearing zone or in different hydrocarbon-bearing zones, and thereby increase the flow of hydrocarbons into the well. Within a single wellbore, there may be one or more zones of interest within various subterranean formations or multiple layers within a particular formation. With particular respect to FIG. 2, a wellbore 202 may traverse one or more zones of interest 204. After drilling, a casing 206 may be lowered into the wellbore 202, and the wellbore 202 may be filled with cement 205 to cement casing 206 in place. After the cementing operation is complete, a perforating tool may be lowered into the wellbore to create perforations 208 through a casing 206 and cement cement and into the near wellbore, in order to access / stimulate the zones of interest 204.

[0017] Stimulation may target single or multiple zones within the well at time through the use of various technologies. For example, stimulation may involve multiple steps such as running a perforating gun down the wellbore to one or more target zones, perforating the target zones, removing the perforating gun, treating the target zones with a hydraulic fracturing fluid, and then isolating the perforated target zones for subsequent production. Conventionally, when targeting a particular zone of interest, operators utilize a geometric stimulation plan wherein perforations are created that are evenly spaced within an interval of the target zone prior to treatment with hydraulic fracturing fluid. Commonly, a geometric stimulation plan uses spacings between the perforation of between about 50 ft and 500 ft. In these instances, the operators may overlook, or otherwise not take into account, the quality (or lack thereof) of the cement job in the interval prior to perforating. This may result in the perforation of wellbore regions that have sub-optimal cementing and ultimately the opportunity for interzonal communication within the near- and far-wellbore. Interzonal communication can reduce the effectiveness of subsequent treatment(s) of the perforated zones with hydraulic fracturing fluid and is generally not desirable in a stimulation operations as it reduced the efficacy of the treatment. In contrast, methods of the present disclosure may use non-geometry or engineered stimulation plans, in which the perforations are evenly spaced within an interval of the target zone. Rather, based on the evaluation of the cement quality (and isolation achieved), the methods may involve more closely spacing a plurality of first stimulation stages to one another with greater spacing between other stages, in order to avoid regions of the well having poor cement quality and/or anticipated fluid communication. Further, it is also envisioned that the number of stimulation stages within a high cement quality region of the wellbore may be increased to better stimulate such region.

[0018] Plug and perforation operations often utilize the installation of plugs inside the casing and/or liner to isolate a target zone for stimulation from the remainder of the well. Plugs and packers may be used to isolate regions of the wellbore to minimize the risk of fracturing fluid by-passing the plug and damaging the wellbore through overflushing at elevated pressures, which can stimulate collateral intervals around the target. [0019] The plug and perforation process may be repeated for all the target zones or a subset of target zones of interest until all the target zones are treated. However, production from multiple fractured intervals may encounter issues with control of the flow of fluids from the formation. For example, in a well producing from a number of separate zones (or from laterals in a multilateral well) in which a high pressure zone may neighbor a low pressure zone, the higher pressure zone may disembogue into the lower pressure zone rather than to the surface, potentially damaging the zone and limiting production. Similarly, in a horizontal well that extends through a single zone, having perforations near the "heel" of the well, i.e., nearer the surface, may begin to produce water before those perforations near the "toe" of the well. The production of water near the heel may then reduce the overall production from the well.

[0020] To remedy possible problems during production, zones may be isolated with various tools such as a packer 210 emplaced on a string of tubing 212. Packers and other isolation elements such as bridge plugs and bull plugs may restrict flow from other intervals while producing from a target interval. In some embodiments, intelligent completions may be used, which involves the use of liner systems, production packers, subsurface flow controls, and subsurface safety valves. Modern completion systems may also incorporate both sensing and control systems, inflow control devices (ICDs), flow control valves (FCVs), pressure gauges, and control lines that may allow users to drain their reservoirs with granularity and may provide an increased feedback regarding fluid movement and reservoir drainage.

[0021] However, the use of isolation techniques and intelligent completion systems within the wellbore may have limited effectiveness in situations in which the cementing job behind the casing that isolates the sections from the formation is incomplete or defective. Following cementing operations in normal course, cementing characterization techniques may not consider cement isolation following emplacement, instead considering solely formation properties, which may not reveal near- and far-wellbore fluid communication between intervals, which can lead to uncertainty with regard to the level of fluid communication between zones, particularly after one or more zones have been perforated during stimulation operations. For example, with particular respect to FIG. 3, defects in primary cementing jobs may result in the formation of channels and microannuli that allow fluid communication through mud channels left in the wellbore 202 or through cracks 304 within the cement 205. Further, depending on a number of factors, such as spacing between fractured intervals and the nature of the formation, vugs and other natural channels may lead to far-wellbore fluid communication 306 as well. Such problems may not be evident following cementing and fracturing operations, and cement characterization is rarely if ever performed prior to subsequent operations. The end result then is that an operator may underestimate the degree of fluid connectivity between perforated intervals, which can result in issues that impact overall production.

[0022] Methods in accordance with the present disclosure may look at zonal isolation quality following cementing operations. In one or more embodiments, well geometry, washout, mud quality, and other variables may be used to create cement quality maps that will be used as a predictive aid to quantify zonal isolation to enable informed design decisions for subsequent fracturing operations. In some embodiments, cement quality maps may be used to prepare fracturing operations such as plug-and-perf operations to enhance production. Cement quality maps may be used as a metric to quantify the zonal isolation confidence factor (ZIF), which may be used with other criteria such as reservoir quality (RQ) and completion quality (CQ) to enable informed design decisions for subsequent fracturing operations in some embodiments.

[0023] In one or more embodiments, formation, casing, and cement evaluation logs such as open hole logs or logging-while-drilling logs, surface measurements, and cementing placement data may be correlated and analyzed in a single workflow to create a cement quality map. It is also envisioned that the cement quality map may be generated from simulations of mud displacement alone or in combination with evaluation logs. In some embodiments, cement quality maps may provide a quantitative or qualitative estimation of the ZIF. In some embodiments, ZIF may be expressed in binary terms, and indicate whether hydraulic isolation is present or not present in a particular interval. In some embodiments, ZIF may be expressed as a likelihood of hydraulic isolation, such as percent confidence of hydraulic isolation in the range of 0% to 100%. In one or more embodiments, the ZIF (percent confidence of hydraulic isolation) of intervals considered to have acceptable cementing quality for perforating in a stimulation plan may be at least 90%, or at least 92.5%, or at least 95%, or at least 98%, or at least 99%, or at least 100%.

[0024] In one or more embodiments, methods may be directed to integrated cement evaluation techniques that may consider a number of factors prior to and following placement of a cement composition within a wellbore. Methods in accordance with the present disclosure may include the creation and design of cement quality maps that may guide the user during the cement formulation and installation process in some embodiments, and/or cement quality maps may be generated from cement logs of an existing cement job and used to design subsequent stimulation operation. Relevant data used to generate cement quality maps in accordance with the present disclosure include open hole data, post placement cement forecasts, cased hole evaluations coupled with pin point stimulation techniques such as BROADBAND PRECISION™ available commercially from Schlumberger Technology Corporation.

[0025] Methods in accordance with the present disclosure may optimize wellbore stimulation operations by analyzing cement isolation data when evaluating stimulation treatment design data and performing calibration based on determined conditions in the wellbore such as pressure and temperature. Methods in accordance with the present disclosure may consider a number of factors regarding emplaced cement jobs such as the quality of the bond to the casing, the anticipated level of hydraulic communication during subsequent wellbore operations including stimulation and fracturing, and the like.

[0026] Methods in accordance with the present disclosure may deliver cement evaluations with a reduced level of uncertainty as to the level of hydraulic isolation and may be used to calibrate formation data with pinpoint accuracy. In some embodiments, methods may include cement emplacement and stimulation design according to the results of the cement isolation characterization.

[0027] In one or more embodiments, cement characterization techniques may be reactive and focused on characterizing an existing cementing job prior to aiding in the design and execution of a stimulation operation. In some embodiments, hydraulic isolation may be characterized in conjunction with other formation evaluation techniques and used to design stimulation operations, including placement of perforations and completions design. Cementing characterization techniques may be developed from open-hole data, post- placement cement forecasts, data from daily drilling reports (DDR), data from cementing operations (e.g., pressure, pump rate and density recordings), simulations of annular mud displacement, and the like. In some embodiments, information obtained from cement hydraulic isolation characterization may be used with other information used in the design of stimulation treatments including open-hole and cased-hole evaluations, wellbore geometry, and formation properties.

[0028] In embodiments in which cement quality maps are incorporated into a wellbore operation design suite, cement quality maps may define the condition of cement sheath behind the casing, such as the degree of hydraulic isolation along the wellbore, and enable users to consider cement quality in addition to relevant formation properties when determining the placement of perforations in fracturing operations. In some embodiments, formation characterization may be calibrated with pinpoint accuracy, minimizing uncertainty related to concerns about cement isolation and allowing targeted placement of perforations and other stimulation treatments.

[0029] In one or more embodiments, field completion plans may be optimized based on updated stimulation/formation data obtained following a cement job. During cementing operations, intervals within a wellbore, such as in a horizontal section, may have insufficient standoff, which may create fingering and channeling of a cement-forming slurry that results in porous structures that enable fluid communication. Cementing jobs may also be modified in some embodiments through the installation of agitators, such as turbolizers, or centralizers within selected zones to minimize channel formation and/or by using practices such as reciprocation, rotation and/or vibration of casing string to minimize mud channel formation.

[0030] In some embodiments, methods may involve selecting fracturing stages along the wellbore above or below plug depths or between cemented sleeves where cement isolation is acceptable and/or avoid regions of anticipated fluid communication. Methods in accordance with the present disclosure may also involve the design of preparatory and remediating treatments for stimulation treatment stages. For example, diversion treatments such as fiber pills or chemical diverters may be used to treat near- or far-wellbore fluid and pressure loss prior to or during stimulation treatments. With particular respect to FIG. 4, a flow diagram of a stimulation design method in accordance with the present disclosure is shown. During the initial stages, formation variables are collected at 402 from a number of sources including information from logging- while-drilling, cuttings analysis, acoustic and radioactive measurements, and the like. Next, primary cementing is installed at 404 following the installation of tubulars and other wellbore equipment. Once primary cementing is completed, the cementing job is characterized at 406 using an appropriate technique such as a cement density log, bond log, variable density log, or ultrasonic cement log (such as USIT of Isolation Scanner™, both of which are available from Schlumberger), and a map of cement quality may optionally be generated. The characterization (and optional) cement quality map may then inform the stimulation job design at 408, which may include importing the characterization (including the cement quality map) into stimulation design software. The stimulation job is then designed, taking into account, for example, the presence of cemented intervals that may have less than optimal levels of expected hydraulic isolation when planning the placement of perforations, frac valves, packers and other equipment to minimize complications in later production operations. That is, if a particular interval has less than desirable cement quality to achieve zonal isolation (but acceptable quality above and below the interval), the stimulation may be designed (or modified) to stimulate an acceptable interval or intervals while avoiding the less than desirable interval. In one or more embodiments, the result of evaluating the formation variables, models, and characterizations that inform stimulation design may be at least one of, avoiding the stimulation of areas with low cement quality, increasing the number of stimulation stages per well, decreasing the spacing between stimulation stages, and more focused/intensified localization of stimulation stages in regions where the cementing has high quality. For example, in regions where the cementing has a high quality, the spacing between stages may be decreased so that the region may be more intensely stimulated thereby increasing the number of stimulation stages. Thus, in some embodiments, a stimulation design may include a region of the wellbore where stimulation stages are more closely spaced than stimulation stages in another wellbore region where the cement quality may not be as high. In one or more embodiments, the stimulation stages in a region of the wellbore may have a spacing of less than about 40 feet, such as by about 20 feet, or even less, to provide for intense stimulation of highly isolated wellbore/formation zones. In some embodiments, when using specific perforation guns, stimulation stages may be spaced less than foot from each other.

[0032] The net effect of informing the stimulation design by evaluating all, or at least some, of the formation variables, models, and characterizations discussed within this disclosure is a more engineered stimulation program when compared with a conventional geometrical stimulation program (i.e., a standardized program that stimulates every X feet). The optimized and engineered stimulation program may lead to increased net production, efficiency, and the realization of increased available reserves within a formation.

[0033] After the quality of the cementing job has been appraised, stimulation operations may be designed using diversion or sequential fracturing techniques such as BROADBAND™ services available from Schlumberger Technology Corporation. Stimulation design may involve setting the number and distance between the perforations for each perforation stage, which may involve placement of perforations or wellbore bridge plugs at regions having acceptable hydraulic isolation.

[0034] During stimulation operations, pressure, temperature, and fluids may be monitored to determine the presence of fluid communication between stages, including near and far field effects. In addition, wellbore clean out may be monitored for the presence of sands in some embodiments. In some embodiments, wellbore cleanout may be monitored for the presence of sand. For example, the presence of sand may be correlated with poor zonal isolation where fluid communication exists above and below an installed packer and may indicate that injected fluids may enter other regions beyond that targeted.

[0035] Other evidence of fluid communication between zones may include data from production monitoring. For example, the contribution of fracture clusters to production may be monitored for poor production, which may be an indication that fluids are being diverted elsewhere during production or that proppant installation was unsuccessful due to leakoff Production monitoring may involve monitoring data from pressure measurements and changes, logging cleanout, production logs, and spinner logs for fluid composition changes such as the introduction of water and brines. In addition, changes in pressure and temperature that may also indicate fluid communication with other zones. In some embodiments, fluid communication may be compared to that predicted from post-job 3D APERTURE™simulations.

[0036] In one or more embodiments, data obtained before and during stimulation operations may be analyzed to determine the existence and level of fluid communication between wellbore stages, including near-wellbore and far-field effects. In some embodiments, fluid communication between wellbore stages may be compared with the results of a number of wellbore evaluation techniques including cement evaluation logs captured from sonic or ultrasonic imaging tools, isolation scanners, cement bonding logs, variable density logs, and the like.

[0037] Methods in accordance with the present disclosure may integrate the analysis of cementing job quality with the stimulating operations, including perforation and fracture design. In one or more embodiments, cement evaluation techniques may be used to develop a cement quality map that may be incorporated into downstream software used to design fracturing operations. Inputs used to develop cement quality maps may be generated from cement quality logs such as cement bonding logs (CBL), variable density logs (VDL), ultrasonic logs and the like. Other inputs for developing cement quality maps may include simulated logs of anticipated cement quality such as synthetic CBL or simulations of mud displacement.

[0038] In one or more embodiments, fluid communication between zones may be quantified, which can provide estimates in the degree of production decrease and fluid diversion rates between zones. For example, the communication between stages may be compared with production data in order to determine the change in overall production rates and whether remediation or intervention is warranted. In some embodiments, zonal isolation quality may be characterized by a bond index describing the degree of cement bonding, defined herein as a "bond index" (BI), between the casing and formation, or between concentric casings in some cases. BI values in accordance with the present disclosure may range from 0%, representing no bond between cement and casing and no hydraulic isolation, to 100%, representing complete cement bonding and hydraulic isolation. Based on application requirements, an operator may subdivide the BI into various subranges such as acceptable and unacceptable. For example, in some embodiments, acceptable BI values may be in the range of 80% to 100%, however, depending on the application, the acceptable range of BI values may be broader or narrower.

[0039] In one or more embodiments, characterization of hydraulic isolation may involve determining the level of cement quality from a number of factors such as bond index, which may be based on CBL readings, along with other information sources such as VDL data, and output from ultrasonic tools that include USIT, ISOLATIONSCANNER™, and the like. In some embodiments, cement quality factors may be derived from open hole or LWD evaluation logs, surface measurements, cementing placement data, and cement bond logs, simulated mud displacement, cement quality maps or a subset or mixtures thereof, and may be correlated and analyzed in single workflow to determine the hydraulic isolation quality.

[0040] In one or more embodiments methods in accordance with the present disclosure may involve the design of single and multistage fracturing treatments that consider formation properties and cement quality behind installed casing. In some embodiments, stimulation treatments may be designed such that fracturing stages are placed in wellbore regions having known degrees of hydraulic isolation behind the casing, including at, above, and below the stage in some cases. In some embodiments, stimulation design may involve sequential fracturing operations such as BROADBAND™ sequential fracturing services available from Schlumberger Technology Corporation, or the use of diverters and other treatments including fiber and/or particulate pills, chemical treatments, and the like to prevent fluid communication between zones during stimulation.

[0041] In one or more embodiments, stimulation design data may be optimized by considering cement isolation data and formation information obtained from log data, pressure measurements, temperature, and other factors. For example, fluid communication between stages, including from near- and far-field wellbore regions, which may in turn be used to estimate cement quality and may inform placement of fracturing stages. Fluid communication measured between stages by, for example, monitoring pressure and temperature changes may be compared with cement evaluation logs such as ultrasonic imaging techniques such as Schlumberger's ISOLATION SCANNER™, cement bond logs, and variable density logs. In one or more embodiments, fluid communication between stages may be analyzed following primary cementing operations from simulations created using specialized oilfield software such as 3D APERTURE™.

[0042] Methods in accordance with the present disclosure may use commercially available cementing software such as INVIZION™, TECHLOG, INVIZION RT, or similar. Wellbore simulations may be conducted on software such as WELLCLEAN II™, WELLCLEAN III™, 3D APERTURE™, CEMENTICS™ or similar that is capable of using inputs that can correlate the state of wellbore (diameter, washouts, and the like) and centralization prediction such as caliper logs, synthetic caliper log, or any other log providing similar information regarding the status of the wellbore.

[0043] In one or more embodiments, wellbore-specific software suites may be used to generate a cement quality map from one or more logs and simulation results. In some embodiments, cement quality maps may be stored in a common database using commercial software packages such a STUDIO™. Cement quality maps may then be accessed by fracturing design software to access the cement quality maps, which may provide a user with information regarding cement quality during the design of single and multistage fracturing treatments.

[0044] In one or more embodiments, methods in accordance with the present disclosure may be proactive, involving the design of a cementing job prior to emplacement and, in some cases, characterization of the cementing job prior to stimulation operations. In some embodiments, cementing operations may be optimized to ensure adequate strength and coverage within an annulus to prevent hydraulic communication between zones, and to withstand the forces exerted on the casing during stimulation.

[0045] In one or more embodiments, cement quality may be estimated predictively, prior to primary cementing. In some embodiments, cement quality maps may be estimated using known properties of the wellbore and installation equipment. Predictive methods of cementing may employ predictive models that generate an anticipated cement quality map from known wellbore data such as wellbore geometry and formation quality, and from equipment properties and variables such as pumping data and fluid characteristics. For example, by using information generated prior to and during drilling operations, the need to take cement logs following cement emplacement may be obviated in some embodiments, which may be advantageous in scenarios in which cement remediation will not be performed prior to initiating fracturing operations. Additionally, predictive methods of generating cement quality maps may also be used to inform the operator whether the actual cementing job may benefit from enhanced best practices in order to adequately accomplish the isolation desired. For example, if a cement quality map that is generated pre-cementing shows that cement quality may be low in in many locations cemented then an operator may be able to pre-emptively take certain actions to optimize and increase the quality of the cementing job. The actions may include performing a more thorough wellbore cleanout prior to cementing, adding more, or optimizing the position of, centralizers compared to what was initially planned, and using a more optimized/specialized cement composition than what was initially planned. The actions may also include adjusting the cementing operations to increase the quality of the cement job (e.g., the cement mixing rate may be adjusted, the displacement pump rate schedule may be adjusted faster or slower), the volume of the spacer or the cement slurry may be changed, mechanical cement plugs (bottom plugs) may be added to keep fluids separate, and/or the temperatures of fluids may be adjusted (e.g. the mixwater temperature or the displacement fluid temperature). In one or more embodiments, an operator may combine information regarding the location of zones targeted for stimulation with the estimated cement quality maps in those particular zones to optimize a planned cementing operation specifically at the targeted zones to ensure that the cement quality in those specific locations is of high quality, while disregarding zones not targeted for stimulation or predicted areas of low cement quality that are not located in zones targeted for stimulation. In these ways, a cementing operation may be optimized to achieve maximum isolation, which may lead to increased production and/or the realization of increased available reserves within a formation. Methods in accordance with the instant disclosure may be applied to horizontal or sub-horizontal wells, including those with multi-stage fracturing completion. During cementing operations, isolation equipment such as packers and plugs may be installed within a wellbore, which may create isolated zones around sleeves in the tubing delivering a cement-forming slurry. Centralizers and other equipment to prevent tubulars and casing from contacting wellbore walls and other tubing may also be installed, particularly in the case of horizontal wells, to ensure adequate cement coverage. In some embodiments, methods in accordance with the present disclosure may utilize cement placement software to optimize the number of centralizers and other cementing equipment to enhance hydraulic isolation, which may include placement of equipment near anticipated perforation sites and other areas to improve cement installation.

[0047] In some embodiments, methods may reduce the total amount of centralizers without decreasing the degree of hydraulic isolation surrounding fracturing stages. Centralizer installation may be driven by plug or sleeve location in some embodiments, which may increase the likelihood of cement isolation in regions in which casing walls will be perforated and more susceptible to fluid communication between poorly cemented zones. In one or more embodiments, design parameters for cementing in accordance with the present disclosure may include having at least 5 casing joints centralized, where 2.5 joints reside above the plug and 2.5 joints reside below the plug, placing centralizers at a density of 2 centralizers per 1 joint. In some embodiments, centralizers and other supports may be provided in regions in which perforations will be located, proving a standoff 2.5 joints below the plug and 2.5 joints above a plug or packer. In some embodiments, a turbolizer may be combined with a normal centralizer, or a single-piece centralizer-turbolizer may be used on the first joint of the interval (if counted from the bottom), and combining centralization with casing movement technique. However, it is also envisioned that an increased number of centralizers may be used, assuming that torque and drag analyses confirm the ability to run the casing to the bottom with the proposed centralizers.

[0048] Other factors that may be considered in cementing design include fluid rheology, torque, and drag when installing equipment such as centralizers and tubing, the presence of completion hardware such as sleeves and screens, fluid flow during mud removal, the need for remedial measures such as pills and diverters, and the like. Cementing operation design may also include factors such as market pricing, material properties, component availability, operating conditions, chemical compatibilities, and the like.

[0049] In one or more embodiments, predictive modeling techniques may include constructing a model prior to a cementing job, calibrating the model to pressure and rate measured or anticipated in a target interval, and interpreting existing formation evaluation logs. In some embodiments, predictive or "synthetic" cement quality maps may be used to select types of cements based on formation properties and wellbore logs. For example, in some embodiments, completions design may be adjusted based on available cement quality or based on performance at pressures and temperatures in a given wellbore interval.

[0050] In one or more embodiments, software suites may be used to analyze logs or simulate cementing results to create synthetic cement quality maps. In one or more embodiments, synthetic cement quality maps may be generated using commercial cementing software such as CEMCAST™ and 3D aperture simulation programs such as CEMENTIC S™, and programs that generate visual log measurements and model the output of cement quality including the MANGROVE™ plugin for PETREL™, all of which are available from Schlumberger Technology Corporation.

[0051] Predictive methods in accordance with the present disclosure may also include computer simulation of mud cleaning prior to primary cementing, which may estimate the degree of mud removal based on existing wellbore equipment, mud characteristics, injected fluid composition and flow properties, and the like. Methods may include simulations of treatments such as post-placement treatment that may include the use of 3D annular displacement simulation software such as WELLCLEAN™ (WELLCLEAN II™, WELLCLEAN III™), 3D APERTURE™, CEMENTIC S™ available from Schlumberger Technology Corporation. WELLCLEAN™ is a numerical cement placement simulator uses computational fluid dynamics to design the process of cement placement. Based on well geometry and trajectory, downhole fluid properties, volumes, pump rates and casing centralization, users may predict the efficiency of mud removal and identify whether a mud channel will remain. In some embodiments, the output from software such as WELLCLEAN™ may also be compared to a map of stimulations sleeves for verification. 3D APERTURE simulates the mud displacement in three dimensional space. The simulator resolves azimuthal flows around the full annulus allowing simulation of gravity-induced segregated flow in horizontal and deviated wells.

[0052] Predictive methods in accordance with the present disclosure may allow users to adjust cement design to ensure hydraulic isolation around zones targeted for completions, minimize fluid connectivity following perforations, and minimize plugging and other production issues. Adjustments to a planned cementing job may include modifying the number of cementing stages, placement of centralizers and other equipment for smart installation techniques, changing fluids design, including rheological properties, thickening time, compressive strength, free fluid, fluid loss and compatibility with other fluids. In some embodiments, a synthetic cement quality map may be generated based on formation properties and cement composition. Synthetic cement quality maps may be integrated into completion design software in some embodiments and used to determine whether the cementing job will be sufficient or should be modified or upgraded.

[0053] In one or more embodiments, predictive methods may consider the initial wellbore condition following drilling operations and before primary cementing. In some embodiments, methods may be used to select and emplace cement within the wellbore. With particular respect to FIG. 5, a flow diagram for an embodiment of a method is shown in which information about the formation is used to design an optimized cementing job. During the initial stages, formation variables are collected at 502 from a number of sources including information from logging-while-drilling, cuttings analysis, acoustic and radioactive measurements, and the like. In addition, information may be collected regarding the type of cement used and other variables such as set time, particle size, the presence of hydration inhibitors or accelerants, the presence of structural additives such as fibers and particulates, and the like.

[0054] At 504, simulations of cement placement within the wellbore are performed using appropriate modeling software. In one or more embodiments, methods of designing a cementing job may also include simulation of torque and drag within the wellbore to determine the ability of running casing to the bottom of the wellbore with the selected centralizer setup. The available information may be used to construct a simulated or synthetic cement quality map at 506 in some embodiments, which may anticipate regions of fluid communication in one or more of a near-wellbore region and a far-wellbore region if present. Cement quality maps are then used to generate a cementing job including, for example, a pumping schedule, cement formulations, spacers, surfactant packages, and orders of component addition. In some embodiments, cement job design may also include the placement of cementing equipment such as tubulars, centralizers, and the like, and may evaluate the hierarchy of friction and pressure within the system, allowing for the adjustment of piping diameter, the types of pumps required to deliver cement slurry, and similar factors.

[0055] Following cementing job design, the job may be performed at 510 and, in some embodiments, evaluated using various logging and predictive techniques to ensure that installation occurred as planned and to verify the quality of hydraulic isolation. In some embodiments, the synthetic cement quality map may also be used to design stimulation treatments at 514 with or without further characterization of the emplaced cement job using cement logs and the like at 512. The stimulation job may then be designed at 514, including the placement of perforations, frac valves, packers and other equipment to minimize complications in later production operations. Following stimulation job design, the job may be evaluated for feasibility and initiated at 516.

[0056] Cement Compositions

[0057] Cement compositions in accordance with the present disclosure may include cements and other settable materials. Cement compositions may include mixtures of lime, silica and alumina, lime and magnesia, silica, alumina and iron oxide, materials such as calcium sulphate and Portland cements, and pozzolanic materials such as ground slag, or fly ash. Formation, pumping, and setting of a cement slurry is known in art, and may include the incorporation of cement accelerators, retardants, dispersants, etc., as known in the art, so as to obtain a slurry and/or set cement with desirable characteristics.

[0058] In a particular embodiment, cement compositions may incorporate a magnesium- based cement such as a "Sorel" cement. Magnesium -based cements are fast setting cements that approach maximum strength within 24 hours of contact with water. While not limited by any particular theory, the cement-forming reaction mechanism is thought to be an acid- base reaction between a magnesium oxide, such as MgO, and available aqueous salts. For example, mixing solid MgO and a brine containing MgCb results in an initial gel formation followed by the crystallization of the gel into an insoluble cement matrix, producing magnesium oxychloride (MOC) cement. Other magnesium-based cements may be formed from the reaction of magnesium cations and a number of counter anions such as, for example, halides, phosphates, sulfates, silicates, aluminosilicates, borates, and carbonates. In some embodiments, anions may be provided by a magnesium salt of the selected anion.

[0059] In addition to MOC cements, prominent examples of magnesium-based cements also include magnesium oxysulfate (MOS) cements formed by the combination of magnesium oxide and a magnesium sulfate solution), and magnesium phosphate (MOP) cements formed by the reaction between magnesium oxide and a soluble phosphate salt, such as ammonium phosphate (NH4H2PO4). Other suitable magnesium cements may also include magnesium carbonate and magnesium silicate cements. In one or more embodiments, magnesium cements may also include combinations of any magnesium cements described herein and those known in the art.

[0060] In other embodiments, the cement composition may be selected from hydraulic cements known in the art, such as those containing compounds of calcium, aluminum, silicon, oxygen and/or sulfur, which set and harden by reaction with water. These include "Portland cements," such as normal Portland or rapid-hardening Portland cement, sulfate- resisting cement, and other modified Portland cements; high-alumina cements, high-alumina calcium-aluminate cements; and the same cements further containing small quantities of accelerators or retarders or air-entraining agents. Other cements may include phosphate cements and Portland cements containing secondary constituents such as fly ash, pozzolan, and the like. Other water-sensitive cements may contain aluminosilicates and silicates that include ASTM Class C fly ash, ASTM Class F fly ash, ground blast furnace slag, calcined clays, partially calcined clays (e.g., metakaolin), silica fume containing aluminum, natural aluminosilicate, feldspars, dehydrated feldspars, alumina and silica sols, synthetic aluminosilicate glass powder, zeolite, scoria, allophone, bentonite and pumice.

[0061] In one or more embodiments, the set time of the cement composition may be controlled by, for example, varying the grain size of the cement components, varying the temperature of the composition, or modifying the availability of the water from a selected water source. In other embodiments, the exothermic reaction of components included in the cement composition (e.g., magnesium oxide, calcium oxide) may be used to increase the temperature of the cement composition and thereby increase the rate of setting or hardening of the composition.

[0062] Cement compositions may also include a variety of inorganic and organic aggregates, such as saw dust, wood flour, marble flour, sand, glass fibers, mineral fibers, and gravel. In some embodiments, a cement component may be used in conjunction with set retarders known in the art to increase the workable set time of the cement. Examples of retarders known in the art include organophosphates, amine phosphonic acids, lignosulfate salts, hydroxycarboxylic acids, carbohydrates, borax, sodium pentaborate, sodium tetraborate, or boric acid, and proteins such as whey protein.

[0063] Embodiments of the present disclosure may be implemented on a computing system.

Any combination of mobile, desktop, server, embedded, or other types of hardware may be used. For example, as shown in FIG. 5, the computing system (500) may include one or more computer processor(s) (502), associated memory (504) (e.g., random access memory (RAM), cache memory, flash memory, etc.), one or more storage device(s) (506) (e.g., a hard disk, an optical drive such as a compact disk (CD) drive or digital versatile disk (DVD) drive, a flash memory stick, etc.), and numerous other elements and functionalities. The computer processor(s) (502) may be an integrated circuit for processing instructions. For example, the computer processor(s) may be one or more cores, or micro-cores of a processor. The computing system (500) may also include one or more input device(s) (510), such as a touchscreen, keyboard, mouse, microphone, touchpad, electronic pen, or any other type of input device. Further, the computing system (500) may include one or more output device(s) (508), such as a screen (e.g., a liquid crystal display (LCD), a plasma display, touchscreen, cathode ray tube (CRT) monitor, projector, or other display device), a printer, external storage, or any other output device. One or more of the output device(s) may be the same or different from the input device(s). The computing system (500) may be connected to a network (512) (e.g., a local area network (LAN), a wide area network (WAN) such as the Internet, mobile network, or any other type of network) via a network interface connection (not shown). The input and output device(s) may be locally or remotely (e.g., via the network (512)) connected to the computer processor(s) (502), memory (504), and storage device(s) (506). Many different types of computing systems exist, and the aforementioned input and output device(s) may take other forms.

[0064] Software instructions in the form of computer readable program code to perform embodiments of the invention may be stored, in whole or in part, temporarily or permanently, on a non-transitory computer readable medium such as a CD, DVD, storage device, a diskette, a tape, flash memory, physical memory, or any other computer readable storage medium. Specifically, the software instructions may correspond to computer readable program code that when executed by a processor(s), is configured to perform embodiments of the invention.

[0065] Further, one or more elements of the aforementioned computing system (500) may be located at a remote location and connected to the other elements over a network (512). Further, embodiments of the invention may be implemented on a distributed system having a plurality of nodes, where each portion of the invention may be located on a different node within the distributed system. In one embodiment of the invention, the node corresponds to a distinct computing device. In some embodiments, the node may correspond to a computer processor with associated physical memory. In some embodiments, the node may correspond to a computer processor or micro-core of a computer processor with shared memory and/or resources.

[0066] Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 1 12(f) for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for' together with an associated function.