Comprehensive Evaluation Method of Drill-in Fluid Damage in Fractured Tight Sandstone Gas Reservoir - A Case Study in the ZM Formation, Tarim Basin, China

Abstract

Fractured tight sandstone gas reservoirs were characterized with narrow pore throats, developed natural fractures, abundant clay minerals, and local ultra-low water saturation, geologic characteristics make those tight reservoirs inclined to strong potential damages. Drill-in fluid may easily invades reservoirs along fractures due to the long soaking time and an overbalanced pressure differential, causing serious formation damage and affecting the economic and efficient development of tight gas reservoirs. An integrated multidisciplinary method was pursued in this paper to accurately evaluate the damage extent of drill-in fluid to tight gas reservoirs. The techniques involve different drill-in fluid damage evaluation methods including core analysis, well logging interpretation, the numerical simulation, and well testing analysis. Results show that the average permeability damage rate through natural flowback after HTHP dynamic damage of drill-in fluid in matrix samples (68.45%–70.70%) were much higher than of fracture samples (44.37%–52.28%). The calculation results of resistivity logging data show that the average filtration invasion depth of drill-in fluid was 1.25 m and the median cumulative filtration invasion depth was 1.26 m. When the simulated bottom hole differential pressure was 3.5 MPa and the simulated soaking time was 150 h, the filtration invasion depth of drill-in fluid was 1.30 m and the lost circulation invasion depth of drill-in fluid was 6.34 m. Afterwards, taking the permeability damage rate and invasion depth as the bridges, the damage skin factor was calculated through Hawkins formula, and the calculated results were in good agreement with the decomposition of well testing results. Finally, an integrated multidisciplinary and multiscale comprehensive evaluation method of drill-in fluid damage in fractured tight sandstone gas reservoirs was proposed, which can serve as a guide for the drill-in fluid damage evaluation in fractured tight gas formation.