Field and Laboratory Verification of Sand-Production Prediction Models

Abstract

Abstract Many completion options are available to minimize sand problems. Formation-strength analysis is essential to selecting and designing the best completion method from these options. This paper addresses the accuracy of several formation-strength models to help those who plan to develop or to use these models in the future. Introduction The many completion options available for completing sand-prone formations include (1) inside-casing gravel packing, (2) openhole gravel packing, (3) selective perforation, (4) a vertical or horizontal well with pre-packed liner, (5) a vertical well with screen, (5) sand consolidation, (6) oriented perforations, and (7) fracture packing. Selection and design of these options depend on analysis of formation strength, strength distribution, permeability, permeability distribution, shale content, fines migration, and grain size and distribution. Of these factors, permeability, permeability distribution, grain size, and grain distribution can be measured with reasonable accuracy; however, most oil companies still have difficulty conducting formation-strength and strength-distribution analyses. The two primary reasons for poor analysis are that mechanical logs available from service companies are not reliable or must be calibrated with other methods and that reasonably reliable numerical models for strength analysis are owned exclusively by several companies. This paper addresses what these formation-strength-analysis models predict, the accuracy required for formation-strength analysis, sources of model errors, model limitations, and methods to improve these models. This paper can be used as a guide engineers who are planning to develop or to acquire these models. Objectives of Formation-Strength Analysis Because some confusion still exists among the users of these models, this section will clarify what needs to be predicted by formation-strength-analysis models. All existing models1-4 predict the onset of sand production from perforation cavities. If the postfailure zone is stable, the model can predict how the failure zone enlarges and may also predict whether the cavity becomes stable if the cavity forms a specific shape after some sand production. However, the model cannot predict sand-flow rate, although qualitative analysis of sand-flow rate can be performed by stress and strength analyses. Under normal conditions, once the well pressure drops below the critical well pressure, sand-flow rate increases significantly with small drawdown. Hence, predicting the critical formation strength where the formation does not produce sand is the primary objective of formation-strength analysis. Currently, sand-flow rate can be determined only by qualitative analysis. Sand-flow rate (not to be confused with onset of sand production) depends on three factors:how much the well pressure is reduced below the critical sand-production pressure, which gives the extent of the postfailure zone;fluid flow rate and viscosity; andcementation because a good indicator of sand-flow rate is whether the formation disintegrates into sand particles (poor cementation) or just slides along a shear band (good cementation) after failure when confining pressure during a triaxial test is approximately equal to effective in-situ stress. Models predicting sand-flow rate that use the discrete-element model are under development but it may take years for field applications because present computers cannot handle the enormous number of discrete elements necessary for realistic field simulations. Furthermore, formation strength varies foot by foot; this requires astronomically large numbers of discrete elements to simulate total sand-flow rate from a formation because the sand-flow rate is affected by the interaction of fluid-flow rates from each interval and vice versa.