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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe North Slope of Alaska has billions of barrels of heavy oil residing in largely undeveloped reservoirs. Despite this large volume of heavy oil in place, the majority of reserves development on the slope to date has been focused on light crude. However over the past 20 years Arco, BP, Conoco and now ConocoPhillips have begun to develop the North Slope's vast heavy oil resource base. Recently a sand/solids control study was undertaken by ConocoPhillips and BP in order to determine the most economic strategy for solids control and well design in future heavy oil developments.The study was integrated across companies, organizations and discipline boundaries in order to include completion, rock mechanics, laboratory research, drilling, reservoir, geological, operations, facilities and field personnel. With this diverse team, actual solids production and solids predictions were investigated from a number of different perspectives. Solids production predictions were made based on core measurements, log analysis, simulators that predict formation failure and sand production rate, laboratory core flow tests, 2 years of field shakeout data, and multiple field measurements of solids production. Probabilistic predictions were then generated based on these investigations rather than deterministic "best guesses" for the economic analysis. These different methods for predicting solids production will be discussed and illustrated in this case study.The study and ensuing strategy determined that sand management or using non-sand exclusion slotted liners and sand tolerant facilities was the highest value development scenario over the life cycle of the North Slope Heavy Oil Developments.

Flow experiments were performed on a ten-inch diameter by fifteen-inch long thick-walled cylindrical sample of poorly consolidated sandstone, with a 1.25-inch diameter borehole. The purpose was to evaluate the effect of changing stress regimes on near-wellbore permeability and liner loading. A one-inch diameter screened liner was installed in the wellbore to preclude sand production. The liner was instrumented with strain gages, in order todetermine stresses resulting from borehole deformation during production. The cylindrical sample was instrumented with pore pressure probes, placed at different distances from the wellbore, in order to assess variation in formation permeability with evolving effective confining stresses and production regimes. The major conclusions of this experiment are as follows:If sand ablation does not occur and if an adequate annular space exists between the liner and the wellbore, load transfer to the liner is mitigated. If sand ablates and fills the annular gap, the complexity of the load transfer mechanism increases dramatically. This has been addressed elsewhere(Abou-Sayed et al., 1995; Willson and Abou-Sayed, 1998).Apparent dilatant behavior in the near-wellbore region leads to permeability increase.An intermediate interval experiences a reduction in permeability, due to stress transfer, following yielding of the inner zone, adjacent to the wellbore. Introduction In unconsolidated formations, it has been speculated that wellbore failure may significantly increase permeability in the failed zone. This could also partially reduce stress transfer to a liner, decreasing the potential for collapse. However, under these conditions, near-wellbore stresses are transferred to an intermediate region away from the wellbore wall. This results in reduced permeability in that intermediate region. Controlled wellbore deformation/failure is required to establish an optimum condition that would lead to overall skin reduction and reduced liner stresses. It may be feasible to manipulate wellbore deformation by "regulating" the annular space between the wellbore wall and the liner (or by using a deformable liner).

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe North Slope of Alaska has billions of barrels of heavy oil residing in largely undeveloped reservoirs. Despite this large volume of heavy oil in place, the majority of reserves development on the slope to date has been focused on light crude. However over the past 20 years Arco, BP, Conoco and now ConocoPhillips have begun to develop the North Slope's vast heavy oil resource base. Recently a sand/solids control study was undertaken by ConocoPhillips and BP in order to determine the most economic strategy for solids control and well design in future heavy oil developments.The study was integrated across companies, organizations and discipline boundaries in order to include completion, rock mechanics, laboratory research, drilling, reservoir, geological, operations, facilities and field personnel. With this diverse team, actual solids production and solids predictions were investigated from a number of different perspectives. Solids production predictions were made based on core measurements, log analysis, simulators that predict formation failure and sand production rate, laboratory core flow tests, 2 years of field shakeout data, and multiple field measurements of solids production. Probabilistic predictions were then generated based on these investigations rather than deterministic "best guesses" for the economic analysis. These different methods for predicting solids production will be discussed and illustrated in this case study.The study and ensuing strategy determined that sand management or using non-sand exclusion slotted liners and sand tolerant facilities was the highest value development scenario over the life cycle of the North Slope Heavy Oil Developments.

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