Sandstone Matrix Stimulation Can Improve Brownfield Oil Production When the Chemistry and Procedures Are Correct

Abstract

Improving oil and gas production from the Brown Fields is now more important than ever to the operating companies, as the oil price remains record high. Matrix stimulation is often preferred as it could generate additional production gain with relatively low level of investment. In the recent acidizing campaign in Brunei, a particular challenge was the flowback of tubing pickling and spent acids, and neutralization of the spent acid on the surface. A series of effective methodologies for the stimulation of offshore multilayer sandstone oil reservoirs was implemented. The chemistry and art of four different acidizing methods involving Tubing Pickling, Bullheading, Diversion and Coiled Tubing placement were used. Stimulation of over forty wells utilizing different acid systems and procedures resulted in noticeably different production gains. The short and long term results are correlated with the stimulation procedures and practices. The present paper describes a comparison of procedures and production gains during these acid stimulation treatments. The cost, logistics and operational constraints due to specific Brunei offshore environment and conditions will also be discussed. Post-treatment production gain is correlated with the efficiency and timing of the flowback process. Use of computer-based virtual laboratory tool for the fluid selection, coreflow laboratory testing for the fluid optimization at downhole conditions and evaluation of fine migration tendencies were investigated before the treatment. The results were compared with the one from other operators in the same environment and reservoir conditions. Review of post acidizing results came up with recommendations and lesson learnt for future campaigns. This effort will certainly enhance the success ratio of the sandstone acidizing treatments. Significance: Developed lessons learnt to increase the success ratio of sandstone stimulations. Introduction Sandstone acidizing is possibility the most complicated stimulation method as it involves complex chemical reactions in the near wellbore matrix1,2,3,4,10,16. Unlike carbonate acidizing where the simulation fluids are used to dissolve the rock around the damage materials, sandstone acidizing aims at dissolving and dispersing the minerals and damage materials in the pores of the reservoir formation. For this reason, the stimulation results using the known standard acid fluid formulations for different reservoir formations and for different wells in the same reservoir can vary widely. Optimization of the sandstone acid stimulation fluids may now be done by numerical simulation of chemical reactions.17 However, it will need a large amount of data not only on detailed mineral compositions of the targeted simulation zones but also on nature, location and extent of the damage materials in the near wellbore matrix. In addition, reaction products due to the reactions of the stimulation fluids with various mills and scales in the tubular and in the pumping equipment can also upset the optimized acid formulation; cause adverse reactions with the formation minerals; and further aggravate the overall complex chemistry involved in the sandstone stimulation process.5 Accordingly, the need to have a proper tubing pickling procedure with proper pickling acid fluid had been repeatedly emphasized. Recent marked effort in this regard included an analytical modeling of the tubing pickling process and its verification using field acidizing job data6,7. The studies showed that the process again involved the three basic mechanisms namely surface reaction, diffusion, and convection, and the art of balancing these three fundamental forces encountered in all sandstone and carbonate stimulation. Generic Sandstone Acidizing Procedure Summarizing the known arts of sandstone acidizing, the following can be served as a recommendation for a generic but minimum standard sandstone acidizing fluid pumping procedure: