Near Wellbore Carbonate Rock Failure Model With Acid-Weakening Effects

Abstract

Loss of near-wellbore rock compressive strength is a damage mechanism related to acidizing in carbonate formations. Non-optimized acidizing job (using improper acid type, acid concentration, acid volume, etc.) may excessively soften the rock and negatively affect the mechanical response of the near-wellbore rock during production. The injected acid dissolves the carbonate rock and increases the local porosity. The porosity is improved variously at different radial distances from the wellbore wall. The weakened rock at different radial distances from the wellbore may become compacted during production time and counterbalance the stimulation effect. In this work, combined wellbore flow, reservoir flow, dissolution, and near-wellbore stress models are utilized to simulate the effects of squeezed acid at different depths of the wellbore and to design the acid to minimize acidizing damage. The models are utilized to analyze acidizing within a horizontal wellbore. The model results show that the overall efficiency of the acid stimulation is primarily a function of the stimulation design parameters (acid type, acid volume, and acid strength, etc.) and the geomechanical characteristics of the carbonate rock.

1. INTRODUCTION

The immediate production improvement and the ability to maintain the production level is governing overall efficiencies of carbonate rock acidizing. New fluids (Al- Anazi and Nasr-El-Din 1998; Al-Taq et al. 2008; Patnana et al. 2013; Zerhboub et al. 1994), new diversions mechanisms (Gonzalez et al. 2017; Safari et al. 2017), advanced modeling capabilities (Kalia and Balakotaiah 2007), and integrated solutions (Abou-Sayed et al. 2005) are developed to maximize production enhancement. The main goal of stated technologies is to develop wormhole dissolution pattern along a completed length of a well. The industry standard technologies overlook stability of the induced wormholes during the expected life of acid stimulation. Wormholes are very high conductive channel departing from wellbore wall which induced by acid reaction with rock. Even though wormholes will increase short term production, their durability especially in high drawdown conditions is not understood.

As the acid is injected into carbonate rock, different dissolution pattern might be developed. McDuff et al. 2010 shows that the dissolution pattern in a clos-to-field condition is greatly function of injection rate and rock type (permeability and porosity systems). Figure 1 shows a wellbore scale experiment (McDuff et al. 2010) that demonstrate operational parameter like acid injection rate can be optimized to achieve the most economical production enhancement.