This paper presents an analytical model for the prediction of the on-set of sand production or critical drawdown pressure (CDP) in high rate gas wells. The model describes the perforation and open-hole cavity stability incorporating both rock and fluid mechanics fundamentals. The pore pressure gradient is calculated using the non-Darcy gas flow equation and coupled with the stress-state for a perfectly Mohr-Coulomb material. Sand production is assumed to initiate when the drawdown pressure condition (i.e. atCDP) induces tensile stresses across the cavity face. Both spherical and cylindrical models are presented. The spherical model is suitable for cased and perforated applications while the cylindrical model is used for a horizontal open-hole completion. For input, the model requires cohesive strength and an internal friction angle that characterizes a Mohr-Coulomb material; preferably predicted using a log-based mechanical properties algorithm in order to generate a foot-by-foot profile of the maximum sand free drawdown for gas wells. The example GOM well illustrates a continuous profile of critical drawdown with depth, providing quick identification of potential sand producing zones. This allows a gravel pack decision to be made in the period between logging and completion. It also facilitates the design of selective perforation programs. The model demonstrates that non-Darcy flow has a considerable effect on the sanding tendency of weak but competent rock, and completion decisions in high gas-rate wells that neglect the influence of non-Darcy flow could be overly optimistic. It also shows that theCDP of a horizontal well with slotted liner is less than that of the corresponding cased and perforated completion. Introduction High-rate gas well completions are common practice in offshore developments and among some of the most prolific gas fields in the world. These fields typically have reservoirs that are highly porous and permeable but weakly consolidated or cemented, and sand production is a major concern. Because of the high gas velocity in the tubing, any sand production associated with this high velocity can be extremely detrimental to the integrity of surface and downhole equipment and pose extreme safety hazards. Prediction of a maximum sand free production rate is therefore critical, not only from a safety point of view but also economically. The unnecessary application of sand controlled techniques, as a precaution against anticipated sand production, can cause an increase in completion costs and a possible reduction in well productivity. However, if operating conditions dictate the need for sand exclusion, such techniques can make a well, which otherwise could have been abandoned or not developed, extremely profitable. The ability to accurately predict CDP is, therefore, critical to optimizing the completion strategy. Two mechanisms responsible for sand production are compressive and tensile failures. Compressive failure refers to tangential stresses near the cavity wall exceeding the compressive strength of the formation. Both stress concentration and fluid withdrawal can trigger this condition. Tensile failure refers to tensile stress triggered exclusively by drawdown pressure exceeding the tensile failure criterion. Veeken et al. 's1 review noted that laboratory and production experiments support the existence of both types of failure mechanisms; with tensile failure predominating in unconsolidated sands and compressive failure in consolidated sandstone. The consensus is that near borehole stresses cause desegregation of the formation while the fluid drag forces provide the medium to remove the failed materials.

No abstract

This paper presents an innovative log-based method of determining pore volume compressibility as a function of pore pressure depletion. The approach considers changes in reservoir stress associated with pore pressure change (stress path) and incorporates constraints that ensure deformations are within elastic bounds. The approach incorporates the effect of stress anisotropy by using elastic moduli derived from stress-strain curves under simulated triaxial loading conditions. The triaxial condition pore volume compressibility was then converted to that of unaxial strain equivalent, which best describes the existing reservoir characteristics. The proposed methodology is particularly useful for predicting pressure-dependent pore volume compressibility where core specimens are either not available or in situations where laboratory measurements are prohibitively laborious and time consuming. For input, the method requires bulk modulus, compressive strength and other mechanical properties that characterize an elastic material, preferably predicted using a log-based mechanical property algorithm in order to generate a foot-by-foot profile of pore volume compressibility. A continuous profile of uniaxial strain pore volume compressibility with depth from log provides quick assessments of pore volume compressibility variations across the reservoir intervals. It is also useful and cost-effective for constraining pore volume compressibility of all the reservoirs penetrated by the well (and logged) but with only limited core data available for calibrations. The example shallow oil well data illustrates that pore volume compressibility decreases with decreasing pore pressure (or increase effective stress). An inverse pore volume compressibility to strength relationship was also observed. It was also observed that pore volume compressibility decreases with increasing porosity until the effective porosity reaches a critical minimum value. At porosity higher than the critical value, the pore volume compressibility increases with increasing porosity. This may suggest that reservoirs with a porosity less than the critical value are more likely to be under pressure drive, while reservoirs with porosity higher than the critical value are more likely to be under compaction drive. Introduction Pore volume compressibility, defined as the relative change in pore volume of a rock with respect to a change in pore pressure, is of fundamental importance in reservoir evaluation and management. It is an important parameter in material balance calculations and water/compaction drive performance studies. Its importance is becoming even more critical with recent fervent deepwater exploration and exploitation activities. These reservoirs, due to their depositional environments, tend to be weakly consolidated and oftentimes over-pressured. Compaction as a result of fluid withdrawal has major implications on reserve estimation, reservoir performance, casing integrity and seafloor subsidence. Due to the high risks and high level of uncertainties involved in deepwater projects, accurate estimations of pore volume compressibility, therefore, play a vital role in the project economics. In conventional depletion type reservoirs without strong pressure supports, reducing the fluid pressure causes changes in the reservoir effective stresses, which subsequently impact the volumetric changes of pore spaces. The engineering parameter quantifying these volumetric variations is pore volume compressibility.

Drilling through depleted sands can result in a multitude of problems such as lost returns, differential sticking, difficult logging and/or not being able to reach the target depth. Often curing lost circulation can be difficult and costly as a result of associated nonproductive time and escalating mud costs. Remedies such as cement plugs, squeezes, expandable liner and casing while drilling can be costly solutions. The use of fluid management techniques, team efforts and proper engineering have lead to the development of an innovative approach to prevent problems and avoid the complex processes of curing mud losses and freeing stuck pipe. This new preventative approach with water-based mud has been applied in several fields, while drilling through a series of highly depleted sands and has proven to be very effective in preventing differential sticking and mud losses. Although operationally successful, the geomechanics and the fluid design resulting in these successes are not well understood. A geomechanical analysis indicates that two mechanisms might contribute to the success:The near wellbore region is turned into a non-porous rock because the particles in the new mud tend to block the pore spaces. The theory of poroelasticity indicates that fracturing pressure is increased by reducing the difference between mud pressure and the pore pressure immediately behind the borehole, which for non-porous rock is zero.Because of this blockage, it is possible that the near wellbore rock strength is increased. This strengthening effect decreases tangential stress and increases fracturing pressure. The geomechanics model can be used to define the operational limits of various mud weights with proper drilling fluids design. This model would enable a consistent and focused approach on drilling fluid design to effectively mitigate massive fluid losses associated with drilling through severely depleted sands or in narrow pore pressure/fracture gradient environments. Introduction Lost circulation has plagued drilling operations throughout history. Generally the types of formations that are prone to lost returns are cavernous and vugular, naturally occurring or induced fractures, unconsolidated sands, highly permeable and highly depleted tight sands. Well known lost circulation control techniques such as bridging, gelling and cementing are typically used, with varying degrees of success. These remedies can sometimes complicate the problems associated with lost returns. Attempts to cure lost circulation can be difficult and costly, especially when considering the associated non-productive time. The lost circulation problems related to drilling through depleted sands are compounded by the low fracture gradient in the sands and the high mud weight required to minimize compressivefailure in the adjacent shales. For depleted sands, the best way to manage lost circulation is to prevent rather than cure the problem. This can be achieved using a combination of a geomechanics and a fluids approach. A literature survey indicates that significant work had been done in this area [1–13]. Lost prevention materials (LPM) were developed to increase fracture initiation or fracture propagation pressure. Recently, a theory of using stress cages to increase fracturing resistance has been developed and demonstrated successfully in the field[2]. Sand bridging or "smearing effect" that is generated by casing while drilling techniques has also been applied[4].

Drilling through highly depleted sands can result in a multitude of problems such as lost returns, differential sticking, difficult logging and/or not being able to reach the target depth. Often curing lost circulation can be difficult and costly as a result of associated nonproductive time and escalating mud costs. Remedies such as cement plugs, squeezes, expandable liner and casing while drilling can be costly solutions and are not always successful. The use of fluid management techniques, team efforts and proper engineering have lead to the development of an innovative approach to prevent problems and avoid the complex processes of curing mud losses and freeing stuck pipe. A newly developed deformable sealing agent can be added to a water-based fluid at 2 to 4% volume. It is a modified liquid insoluble polymer that is designed to reduce pore pressure transmission by internally bridging the pore throats of the low permeability sands and shale micro-fractures. These bridging and sealing characteristics will help protect the formation where lost circulation may be encountered. This effective bridging enhances the effective rock strength, hence increasing the formation fracturing resistance. A geomechanical analysis indicates that two mechanisms contribute to the success:The near wellbore region is turned into an altered rock because the particles in the new mud tend to block the pore spaces. Stress analyses using rock mechanics theory indicate that fracturing pressure is increased by increasing the tangential stress around the borehole resulting from the ‘enhanced’ mechanical properties of the altered zone.Because of this blockage, it is envisaged that the near wellbore rock strength is increased. This strengthening effect increases the bulk and tensile strengths of the altered rock and increases the fracturing pressure. This paper will highlight field case histories supported by preliminary laboratory work and geomechanical studies indicate that mud losses associated with severely depleted tight sands can be reduced with the use of the newly developed deformable sealing technology. Some of the field accomplishments of this "internal mud cake" as a bridging/sealing approach are: improved drilling curve, lower well cost, stable and gauge hole, reduction in mud losses and differential sticking and reduction in NPT Introduction Drilling through shallow, highly depleted sands is prone to severe lost returns and differential sticking. Lost circulation and differential sticking problems related to drilling through depleted sands are compounded by the low fracture gradient in the sands and the high mud weight required to minimize compressive failure in adjacent shales. Therefore, drilling deeper to reach new targets in mature fields is becoming more attractive and often presents technical and economical challenges. Designing a well in such complex geological settings often results in additional casing intervals and/or the use of expensive expendable liners or casing while drilling. Typical lost circulation control techniques are costly and may not be applicable. It is better to manage lost circulation by preventing the problem, rather than attempting to cure it. A literature survey indicates that significant work has been conducted on wellbore strengthening[1–13]. Prevention methods have been developed to increase fracture initiation and fracture propagation pressure. Recently, a theory of using stress cages to increase fracturing resistance has been developed and demonstrated successfully in the field[2]. Sand bridging or "smearing effect" that is generated by casing while drilling techniques has also been documented[4].