Modelling of the Collapse of Unconsolidated Rock Formations During Production

Abstract

Abstract The definition for the maximum allowable drawdown for sand free production in uncased and cased wellbores is discussed herein. An analytical procedure is developed to evaluate the effective stresses around wellbores during production. This methodology takes into account a general stress field as production. This methodology takes into account a general stress field as well as wellbore inclination and azimuth and allows for the mechanical failure of the formation. A number of computational experiments are carried out to demonstrate the effects of the in-situ stress ratios and formation cohesion upon the maximum allowable drawdown for sand free production. production. Introduction Large oil and gas reserves stored in rock formations known as unconsolidated or weakly cemented sands are known to exist in several areas. In general, these low-strength rocks do not support the stresses that are generated during the flow of the produced fluid through the formation and eventually faliure will occur causing the breakout of the rock. The positioning of the failed rock mass around the wellbore dictates the conditions for the removal of solids from the formation and its transport by the produced fluid. The transport of solid by the produced fluid is known to be the cause of a number of undesirable problems such as damage to submersible pumps, plugging off the case perforations, and, eventually, total invasion of the production column. In order to address the questions associated with the production of solids and its impact on the economics of the reservoir expolitation, it is mandatory to have a scheme to predict whether and when a given rock formation subjected to a given production rate will produce solids. Ideally, for the purpose of selecting a proper completion scheme and of planning the well production strategy, the completion engineer should be planning the well production strategy, the completion engineer should be able to define the maximum allowable drawdown that guarantees sand free production throughout the operational life of the well. This paper production throughout the operational life of the well. This paper discusses some of the necessary steps involved in the determination maximum allowable drawdown for sand free oil production. Initially, a conceptual model for sand production is presented in which both the sufficient and necessary conditions for sand production are highlighted. This conceptual model is used as a background to review critically the existing sand production prediction tools as well as to formulate guidelines for the production prediction tools as well as to formulate guidelines for the development of new analytical prediction methods.