Predicting Sand Production in U.S. Gulf Coast Gas Wells Producing Free Water

Summary A study was undertaken to develop a statistical model capable of predicting the sanding of U.S. gull coast gas wells that also produce free water. Available models predict sand production when only hydrocarbons are being produced, but none of the current models apply to gas wells when there is free-water production. Field data from gas wells and log-derived properties of reservoir rock were used to construct a usable model. This new model, which differs from current models by using the previously developed "dry" model as an input in the statistically generated new "wet" model, has provided a practical tool that is based on failure criteria but validated by field data. The model presented here fits field data of water-producing gas wells better than the one currently available. When the optimum decision concerning gas production rate and the need for sand control can be made at the time of initial completion, more profit can be realized from the project. It is hoped that this model can aid in the prevention of formation and equipment damage in gas wells owing to sanding. Introduction Numerous solutions to halt sand production from oil and gas wells have been attempted, with various degrees of success. The most prevalent remedy is the gravel-pack completion, which blocks the influx of loose sand with specially selected gravel held in place by screens (Fig. 1). This method is particularly expensive but not nearly as costly as losing a producer. Therefore, it is vital to know whether a well will produce sand before it is placed on production. The economic implications of sand problems are critical enough to require continuous improvement in sand-control techniques and sand-production prediction methods. Production problems attributed to sand production are sand fill-up and bridging inside the wellbore that shut off production; erosion damage to downhole tubular goods, safety valves. and artificial-lift equipment; downhole casing and formation damage that sometimes causes premature abandonment of completions, sand accumulations in surface lines and equipment. and abrasive wear on surface chokes, valves, and pipes. In addition to the cost of these problems, unexpected pipe leaks or equipment failures present serious safety and spill hazards in offshore and inland water locations. The history of predictive models for sand control is relatively short. Stein and Hilchie, who introduced the first significant technique, related the formation's shear strength to the well's sand production. Data from sonic and density logs were used to relate production of a sand-producing well to that of a well under study. The limitations of the method are thata well must be completed and tested until it produces large quantities of sand before reliable results are obtained,the problems of pressure depletion and water production were not addressed,the well is investigated at one point in time without any method of extrapolating into the future, andcompletions are assumed to be in clean sands without skin effects. Tixier et al. reported the development of the mechanical properties log, which was basically a log-derived model. Their empirical correlation implied that a threshold for sanding existed at G/cb = 0.8 × 10 psi [5.516 × 10 mPa], where G is shear modulus and C is bulk compressibility. This correlation can state only whether sanding will be a problem at current conditions and has shortcomings similar to those of Stein and Hilchie's model. A major drawback is the lack of quantitative information. The method states whether a well will be a sand producer, but a maximum sand-free rate cannot be calculated from the given ratio of G/cb. The "sand strength" logs was developed in 1981. This model, unlike the previous two, relates sand production to the stress levels existing around the near-wellbore reservoir rock. Mohr's circle stress analysis technique is the heart of the method. Log-derived elastic rock properties are used to obtain compressibility constants and in-situ values of stress around the borehole. This method has been gaining acceptance in the industry as a reliable predictor of sand production in hydrocarbon producers that do not produce significant water volumes. A study of 16 U.S. gulf coast wells concluded that Mohr's failure analysis technique with a 200-psi [1.378-MPa) safety factor would have been a viable method for making sand-control decisions. Although Mohr's stress analysis technique, referred to here as the dry model, is good when applied to wells with no water production, its limitations were quickly realized. Although the experience to date has been predominantly with gas and oil production, it is believed that production with a high water cut may require higher intrinsic shear strength. This threshold criterion of water-producing sands has yet to be established. Weissenburger et al. also realized the need for a system to predict sand production. An engineering system provided an iterative pathway to integrate rock mechanics, geology, logging, and reservoir-management information. Morita et al. provided a numerical model and a parametric study of sand-production prediction without the effect of water production. An analytical method for predicting sand production in gas wells making water is not currently available. JPT P. 1336^