In addition, a complete set of data would include a dipole sonic log, confined compression and tension tests on core, and fracture gradient. With this information, a synthetic shear velocity log is generated, rock strengths are estimated from correlations, and in situ stresses are estimated from rock properties. The minimum information required for the analysis are a log analysis, including sonic and density logs, gas properties (temperature, pressure, and gravity), and estimates of reservoir area, thickness, and depth. Our goal was to have a method that would allow gravel-pack decisions to be made in the time period between logging and completion, typically a few days. We chose the analytical solution over a finite-element program because, in our experience, available data, timing, and resources generally do not justify the complexity of numerical simulation. The tools are an analytical solution for hand calculation, a log analysis program for foot-by-foot analysis, and a spreadsheet for accelerating the hand calculation. The primary components of the method are prediction of rock strength, calculation of maximum drawdown for perforation stability, and calculation of reservoir failure. The technique described in this paper was developed to work even with minimal data, while also taking advantage of extra data that might exist. In some fields, data, including both logs and core tests, are plentiful but, in most, only minimal data are available for making this decision. Prediction of maximum sand-free production rate provides information for sand-control decisions and allows maximization of rate in those wells that are completed without sand control. The method also addresses how allowable drawdown changes with reservoir depletion, which existing models do not consider. In addition, it allows for higher drawdowns than those permitted by the shear failure criteria, up to the drawdown that would produce tensile stresses at the perforation face. It models pressure gradients in the reservoir instead of assuming that all the pressure drop occurs at the perforation face. The method presented differs from commonly used log-based sand-prediction models in two important ways. Field production data are presented and compared with theoretical predictions as well as with predictions based on traditional shear failure theories. The prediction method was incorporated into a simple log analysis program that facilitates quick identification and analysis of potential sand-producing zones, and this program is discussed. Rock strengths determined by core tests and log correlations are compared. In the fields addressed in this paper, rock strength was determined in one or more ways: core testing, log correlations by use of direct shear velocity measurements with the dipole sonic log, or log correlations with traditional sonic logs and calculated shear velocities.
The method has since been applied extensively worldwide by Arco. This paper presents a method for predicting sand production in gas wells and the results of applying that method to 13 fields in the U.S. The ability to predict at what point sand problems will occur is useful. Also, wells that do not initially require sand control may later become sand producers. In unconsolidated sand, the decision to gravel pack is usually clear however, the decision is harder in weak rock because the need for sand control often depends on the desired drawdown or production rate. Sand production from weak, but competent, rock as a result of high production rates is a growing concern.
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