Return Permeability Tests and Relevant Laboratory Evaluation to Optimize Remedial Design for Reviving Productivity of Cased Hole Well Damaged by Oil-Based-Mud
Authors: Yonebayashi Hideharu , Miyagawa Yoshihiro , Watanabe Takumi , Kurokawa Susumu , Teshima Satoshi , Richmond Michael H. 
Published: November 2017

Abstract
An ultimate objective is specified to overcome the oil productivity deterioration in a cased hole producer that was drilled throughout by means of oil-based mud (OBM). Significant problems can be caused by OBM invasion into the reservoir and/or contamination by the OBM interacting with completion/formation brines in the hole. In this study for designing an optimum remedial program, a series of laboratory works were conducted using reservoir cores to understand accurately the damaged conditions/location at the site. For this study of a cased hole well drilled throughout by OBM, an unexpected low oil productivity was assumed with non-negligible water production. Multiple reasons were considered such as near-wellbore formation damage (including wettability change, emulsion blockage, etc.) or plugging in certain sections of the cased hole. To identify the main reason, return permeability tests (RPTs) were performed for several contaminating combinations: OBM versus formation/completion brines, or reservoir oil. Comprehensive evaluation determined the mechanism that caused the problem. Simultaneously, a bottle-test (BT) was conducted to evaluate if a micro-emulsion wellbore remediation fluid would dissolve the contaminated material.
The RPTs indicated that minor OBM invasion occurred for the formation-brine-saturated core (RPT-1), and significant invasion for the core saturated with base oil (RPT-2). In the RPT-1 core, most OBM was trapped at the inlet-side core. This reduced the invasion, and the return permeability could maintain 93% of its original value. In contrast, the inlet/outlet surfaces of the RPT-2 core were cleaner than those of the RPT-1 core. This revealed that the OBM had invaded the RPT-2 core. Consequently, the return permeability dropped down to 32% of the original value.
The BT showed that the multifunctional single-phase emulsion fluid could dissolve the sticky, contaminated material caused by mixing OBM and the completion brine. The contaminant strongly adhered to the glass tube wall and was quickly detached, creating flocs that could not be removed by solvents such as toluene. Contact angles on the tested cores were measured to understand measured area.
These findings were useful for optimizing a remedial program: a two-staged operation consisting of removing contaminants from the cased hole followed by soaking for dissolving the invaded OBM near the wellbore.
This study provides the valuable findings of laboratory evaluation of OBM-related damage in the cased hole well. The clarification of problem mechanism and location could be essential data for appropriate remedial design. A single test that screened effective dissolution-fluid was a part of remedial job design, but not enough for optimizing the design. This comprehensive evaluation approach of the RPT and BT demonstrated the most likely bottomhole/near-wellbore situations to be taken into account.