Numerical Evaluation of Geomechanical Parameters Affecting Productivity Index in Weak Rock Formations-Part 1: Theory
Authors: Vaziri H.H. , Xiao Y. 
Published: March 2010

Abstract
An elasto-plastic, fully-coupled fluid flow and deformation finite element model is proposed for modelling development of both shear and tensile failure; the latter representing the volume of the solids that are in a liquefied state and hence ready to be transported and produced. The model takes account of the changes in permeability in accordance with changes in the stress state (or porosity), including failure. This feature allows for computation of the changes in skin as the reservoir is depleted and sand is produced. One of the key aspects of the model is its employment of the modified Mohr-Coulomb failure envelope along with several numerical algorithms to effectively simulate shear and tensile failure in a numerically accurate and stable form. The modified failure envelope provides a means of capturing the strong non-linearity in the failure envelope for the weak rock formation around the wellbore before solids production. In that state, the material is practically in a disaggregated state due to shearing, yet it has a high density. The most important feature of this model is its capability to capture the episodic nature of drawdown induced sand production. Verification and application of this model to a well-documented openhole cavity completion in coalbed methane in the San Juan Basin is shown in a companion paper.

Introduction
The conventional practice in oil and gas recovery projects is to minimize sand production by installing various sand exclusion measures. While in a number of field cases this is justified, often times such measures are employed prematurely, resulting not only in redundant expenditure but also diminishing the well's productivity due to the inevitable plugging of filters(1). There are many field cases that show seepage or drawdown-induced sand production can be mitigated by a careful control of the flow rate, which means that in the early stages of a well's life there is no need to use sand exclusion measures. Over the past decade, an increasing number of studies have been performed that point to the significant advantages of promoting sand production(2–4). For instance, it is reasonable to suggest that creating mini sand bursts can remove or clean the near wellbore skin damage, which may increase productivity by as much as 30%. By using well-established geomechanical concepts, such events can be controlled quite reliably, thus minimizing the risk of triggering large or ongoing sanding.
A far more effective way of boosting production is to intentionally create massive sand production for reservoirs that have favourable geological properties and characteristics so that a stable cavity and enhanced porosity region in the vicinity of the wellbore face can be achieved. In a number of fields, cavitation has been instrumental in improving the performance of the reservoir by several-fold(5). Cavitation, however, has not been effective in all cases. It is the objective of this paper to evaluate the geomechanical parameters and properties that influence the success of openhole cavity completions.