Sand Clean Out and Acidization of Predrilled Liner Horizontal Hole Sections: A Field Experience

Abstract

This paper describes sand cleanout / acidizing operations of four horizontal pre-drilled liner completions in the UU field, offshore Nigeria. These wells were drilled and completed concurrently with similar completions in 1996. Production from the wells declined after four years. The wells were studied using production history curve analysis and nodal analysis techniques. Analysis indicated that the most probable cause of the decline in productivity was plugging of the completion with solids/deposits. Detailed analysis and flow testing were carried out on the available core samples from an analogue well to determine the mineralogy of the possible plugging solids and the most effective treatment recipe to be used to remove the plugging materials. Based on the results of the core analysis, sand cleanout /acid stimulation operations were executed on the subject wells, using coil tubing and specialized coil tubing tools. The operation resulted in a 300% increase in productivity, and was both a technical and economic success. Introduction The primary objective of the stimulation program was to increase the productivity of the wells on the six-well platform known as UU G. Among the six wells on this platform, two of the wells were selected as re-drill candidates. The other remaining four wells were identified as stimulation candidates. These four wells were suspected to be plugged with fines, organic materials and possibly scales. Detailed core analysis, which included X-ray diffraction analysis, scanning electron microscopy, flow testing of the core sample with various BJ acid recipes and effluent analysis by ICP cation concentration determination, was carried out on a core sample from an analogue well. Results of this analysis suggested the possible plugging material and the effective acid recipe to be used to clean out and stimulate the horizontal section of the subject wells. The acidizing operation was designed to be preceded by a sand wash procedure, whereby a switchable jetting nozzle was used to remove sediments from the horizontal drainhole section. Flow tests carried out on the core samples indicated that the fines plugging the pre-drilled liner / reservoir could be removed by pumping a retarded HF acid system. The treatment train also included a fine stabilization additive, in a bid to prolong the effectiveness of the acid operation. The acid was placed in a controlled circulation flowing mode in order to minimize stimulation of the water zones/leg and maximize stimulation of the plugged zone. A Coil Tubing deployed rotating jetting nozzle was used for purposes of mechanical diversion and optimization of the jet impact force. The four wells selected for stimulation were completed with 4.5" predrilled liner across the drain section. In two of the completions, the tubing was stabbed directly into the PBR of the liner hanger/packer at the top of the predrilled liner, while for the other two wells, a packer was run with the tubing set at about 50ft from the top of the liner packer. This left a gap between the end of tubing and the top of the liner hanger.