A Deformable Material for Sand Consolidation

Most sand-consolidating plastics require the sequential injection of at least four different treating solutions to either catalyze the plastic or to prepare the formation to receive the plastic. In addition, the formation is consolidated into a rigid, brittle mass of less strength than is frequently necessary to withstand down-hole stresses. A theoretical study and subsequent research to remove these limitations led to the development of a moisture-cured urethane elastomer which consolidates the sand in a tough. rubbery matrix that will undergo considerable movement without sloughing. This property allows the consolidated mass to deform around the casing so that the casing itself withstands the stresses. A typical treatment consists of the injection ofa small volume of kerosene or diesel oil to establish an injection rate,the polyurethane solution, anda diesel oil overflush to establish permeability. The polyurethane is catalyzed by the formation water and therefore exhibits relatively unlimited placement time. To date, this technique has been used in six wells experiencing severe sand problems in the Gulf Coast area. Three wells have been unqualified successes, one was abandoned prior to full evaluation and two failed to restrain the sand. Introduction Sand problems are associated with loosely consolidated or completely unconsolidated formations. Such formations are unstable and fail when exposed to stresses caused by fluid velocities imposed while producing a well. This failure causes sand to slough into the well and create a restriction which severely impairs its productive capacity. Accordingly, successful restraint of unconsolidated sand in producing wells has been the object of intensive industry-wide research during the past two decades. During this period the most reliable method of controlling sand has been the gravel-packed screen which has a reported success of 85 to 90 per cent of all wells in which it has been tried. However, this success ratio can be misleading if it is not realized that many of these installations require periodic remedial operations to maintain top allowable production. This limitation, coupled with the 10 to 15 per cent of outright failures, indicates the need for more effective sand control measures which are applicable for both single and multizone completions. The high coast of Gulf Coast offshore operations also emphasizes the need for a satisfactory solution to the problem. The ideal sand-restraining device would be one which holds the loose sand outside the casing and does not seriously reduce the flow capacity of the well. Thus, it follows that some method of formation consolidation is needed. Phenol-formaldehyde plastics have been available for sand consolidation for about 20 years; however, the early plastics would not bond effectively to the sand. Severe flow capacity reduction and strength development insufficient to support the reservoir stresses and flow velocities imposed also contributed to their slow acceptance. Poor bonding characteristics, flow restriction and low strength development exhibited by the phenol-formaldehyde plastics have been markedly improved in recent years. Epoxy resin systems have also been introduced which exhibit high strength and high flow retention properties. However, most sand control plastics now available to the industry require complex sequential injection procedures and/or have limited pumpability because of the catalyst systems used. It was postulated that the success ratio of plastic consolidation techniques could be improved by achieving two objectives:simplify the treating procedure by reducing the number of sequential injections required, andcatalyze the plastic in situ by formation fluids. The latter objective increases the length of time during which the plastic can be placed allowing more flexible treatments, and also maximizes the possibility that all plastic injected will be catalyzed. With these objectives in mind a search for applicable materials was begun. A theoretical study of stresses around a wellbore was also made to define factors which enhance sand consolidation results. Theory A mathematical treatment of elasticity and plasticity equations bearing on the sand control problem is included in the Appendix. It is shown that to prevent yielding around a wellbore or perforation, compressive strengths in pounds per square inch approximately equal to the well depth in feet are required. JPT P. 306ˆ