Laboratory Preparatory Work of Cased-Hole Clean-Up as a Part of Entire Remedy Program in Removing Damage Attributed to Oil-Based-Mud
Authors: Yonebayashi Hideharu , Miyagawa Yoshihiro , Watanabe Takumi , Kurokawa Susumu , Teshima Satoshi , Yamada Ryo 
Published: January 2018

Abstract
An ultimate goal is to overcome the oil productivity deterioration in a cased hole producer drilled throughout by means of oil-based-mud (OBM). However, problematic location has not been exactly identified whether in formation by OBM-invasion and/or in hole plugged by contamination. Therefore, in our previous study, the return permeability tests were carried out and revealed two potential damaged region: formation and/or cased/perforation holes. The objective in this paper is to screen appropriate solvents to remove contaminants from the cased hole and perforation hole.
Unexpected low oil productivity with non-negligible water production suspected multiple reasons such as near-wellbore formation damage and/or partial plugging in certain sections of cased-hole and perforation tunnels. To address the issue of low productivity under the situation of uncertainty in the damaged region, chemical treatment was planned in two steps: the first step for damage in the wellbore and perforation tunnels, and the second step for near-wellbore formation damage. Before the first step of the treatment, effective solvent should be screened for removing contaminants from cased-hole. Two solvents ("A" and "B") and xylene (for comparison) were evaluated by dissolution tests at reservoir condition. OBM and KCl-brine were injected into a dummy core holder to form synthetic contaminant in a mixing cell, and then solvent was injected to dissolve it at several mixing conditions. Each effluent was observed to compare dissolving process.
Three mixing conditions were assumed by adjusting injection rate of each fluid: (1) OBM with KCl-brine and (2) solvent. This mimicked a series of contact situation in deviated hole. By solvent type, both candidates showed good dissolution abilities because most of sticky immovable contaminants were discharged from the dummy core holder. After disassembled to check on the inside, a little amount of residue was observed around the outlet cap. Moreover, the residue was not sticky and easily removed. According to visual inspection of dissolved contaminants in effluent, the solvent-A made flocs that had floatable characteristics. On contrary, the solvent-B extracted oily component followed by forming flocs. However, the solvent-B also formed fine silt that had sinkable but flowable characteristics. By injection rate, dissolution process was progressed identically in any step for the solvent-A whereas step-wisely for the solvent-B. Finally, it was concluded that both solvents could dissolve contaminants effectively. Comparatively speaking, the solvent-A seemed to be able to work regardless of the contact conditions rather than the solvent-B. In case of considering clean up flow, the solvent-A might be better than the solvent-B because of more of floatable flocs formed.
This study provides practical remedy program and laboratory workflow. Tested solvents are commercially available, therefore, the operator can request the suppliers to perform a laboratory evaluation. However, such independent deliverables can allow insufficient comparative evaluation for screening the best candidate. In-house study demonstrated variation of dissolution process between two solvents. Its result can contribute viewpoints of flocs floatability that is sensitive in case of considering not only dissolution ability but also clean-up flow.