Application of Dynamic Underbalance Perforating Technique on a Drillstem Test String in Combination with a Nondamaging Viscoelastic Surfactant-Based Fluid Loss Control Pill: Case Study from Western Siberia

Abstract

The main challenge of perforation completion is to provide communication between the wellbore and the undamaged reservoir through clean perforation tunnels. To achieve this, two key technologies were implemented for tubing-conveyed perforation (TCP) in workover wells in a mature field in Western Siberia. First, the application of dynamic underbalanced perforating achieved clean perforation tunnels, resulting in improved production. Second, the placement of a nondamaging viscoelastic surfactant-based (VES-based) pill after perforation avoided reservoir damage during the well kill operations through clean perforation tunnels. Three wells in the mature Van-Egan oilfield were selected as candidates for the perforating project. Two wells were previously perforated with standard charges, and the third well was to be perforated across a new layer. Deep penetrating charges were used to bypass near-wellbore damage and maximize potential results. A customized TCP test string for 6 5/8-in. (168-mm) casing was implemented with two sets of gauges installed at different depths. The gauges included a downhole shut-in valve for a closed chamber test, a production valve for the inflow surge, and a fill-up valve for improved run-in-hole procedures. The TCP test string was designed to conduct a perforation inflow test to estimate post-perforation skin factor, productive zone permeability, and reservoir pressure. For the third well with 5 3/4-in. (146-mm) casing, a modified toolstring was used due to operational restrictions. After perforation, a novel, nondamaging solids-free VES-based fluid was used as a fluid loss control pill to protect open perforation tunnels during the subsequent workover operations. Several critical parameters such as permeability, bottomhole temperature, possible scale and emulsion formation, specific gravity, and stability time required were assessed to find the best pill formula for each well. This approach successfully protected clean perforating tunnels against damaging kill fluids for 3–5 days of workover operations. Results showed negative skin values for reperforated intervals and slightly positive skin values for the new perforated interval. Improved loss control was reported after spotting the pill. Zero losses were achieved in a low-permeability reservoir; loss rate observed in the reservoir with high permeability was lowered by more than a factor of 4. Post-job production data showed an increase in oil rate of up to 2.5 fold compared with the preworkover rate.