Restoring Injectivity of Wells Drilled in Sour Carbonate Formations

Abstract

Saudi Aramco has embarked on developing an oil field located in the Northern area of Saudi Arabia. This field is characterized by the presence of tar-mat zones that are present at the base of oil-column. These zones form a barrier which physically isolates the producing zones and the injection water. This paper addresses the challenges faced in maintaining the injectivity of water in this particular field. Open-hole water injectors with extended reach (ER) were completed horizontally at the bottom of the producing zones and just-above the tar-mat zones. The typical open-hole lengths in these wells are between 3,000 -7,000 ft. Challenges were faced in drilling these wells in a populated area in the northern area of Saudi Arabia and being stratified, tar-mat was encountered in the open-hole sections of these wells. Tar content varies inconsistently around these injectors. This limits the injection rate of these injectors initially to as low as 600 BWD at 2,400 psig. The low-injectivity of these water injectors is attributed to the damage induced by the drilling mud (water-based mud) and the presence of tar and asphaltenes in the injection zone. Acid stimulation treatments of these wells were needed to reach the target injection rates. Furthermore, this particular northern field in Saudi Arabia is sour and located inside a highly populated area and as such, environmental regulations prohibit any well flow back post acid treatment and no flaring is allowed. To overcome these challenges a tailored acid treatment was designed. It included a preflush of an aromatic solvent, then stages of 20 wt% HCl regular and emulsified acids. To achieve better acid diversion in horizontal wells, stages of a surfactant-based acid system were utilized. Along with tailored acid treatments, new methods to place the acid in the zone of interest were also employed. More than 20 extended reach horizontal injectors were successfully stimulated in a sour carbonate oil reservoir. Pre and post treatment injectivity tests were performed using down-hole gauges in order to evaluate the efficiency of the acid treatments. This paper will discuss challenges faced during stimulation of water injectors in this sour heterogeneous reservoir, fluid selection, design criteria, field treatments, lessons learned, and results of the acid treatments. Introduction The development of the northern oil field by Saudi Aramco necessitates 30% of the newly drilled wells to be water injection or water disposal wells. The water injectors are completed with 6 1/8-inch open hole horizontal wells with 3,000 to 7,000 ft open hole section for maximum reservoir contact (MRC) purpose. Most of these wells are considered as extended reach wells.1 Tar zones are present in Field Q, which produces oil mainly from three carbonate reservoirs: AC, AD, and LF.2 The three carbonate reservoirs have different properties. Table 1 provides the average permeability, porosity and grain density of the three reservoirs in Field Q. Reservoir AC has the highest permeability, whereas reservoir LF has the lowest permeability. The grain density of reservoir AD was the highest, indicating higher dolomite content in this reservoir. Peripheral water injection has been used to maintain reservoir pressure and oil production. Tar zones with almost zero permeability act as a barrier that hinders water flow in the reservoir, and as a result, the pressure in some areas will decrease with time. Injectivity tests performed after completion of these injectors showed very poor injection rates, which were attributed mainly to the damage induced by the drilling mud (water-based mud) and the presence of tar in the injection zone. Acid stimulation treatment campaign was launched in 2005 targeting some of the water injectors to increase the injection rate and maintain the required reservoir pressure by peripheral water injection. The following challenges had to be overcome to effectively stimulate these injector wells:Long open hole length ranging from 3,000 to 7,000 ftSignificant changes in the reservoir permeability, which varies from 50 to 1,000 mDSour environment (16 mol% H2S)High tar contentRestriction on well flow back before and after the acid treatments These challenges were addressed by carefully designing acid treatments. The objectives of the present study are to:Discuss factors that affect the design and execution of acid treatments, andAssess acid treatments based on field data.

Development and Field Application of a New Hydrogen Sulfide Scavenger for Acidizing Sour-Water Injectors

“…Most of these wells are considered as extended reach wells. 13,14 Injectivity tests performed after completion of these injectors showed injection rates below expectations, and were attributed mainly to the damage induced by the drilling mud (water-based mud), the presence of tar in the injection zone, and iron sulfide species. The latter was formed because the injection water contains 2-5 mg/l total iron and the reservoir is sour.…”

“…13,14 In the first few acid treatments, Scavenger A was used. This scavenger reacts with hydrogen sulfide and produces an oily material, which may affect the relative permeability to water and wettability in the critical near well bore area.…”