Features of Field Development with Tight Carbonate Reservoirs by Waterflooding

Abstract

The Bulak field is a unique oil field located in the South-Turgai basin. It consists of heterogeneous carbonates. Throughout the field, the reservoirs vary from highly fractured clean limestones with strong aquifer to very tight limy claystone with low amount of bed-bound fractures and no water production. Active infill drilling followed after the exploration stage revealed uncertainties related to the main issues of development, i.e., location of new wells, their trajectory, EOR techniques and most importantly, waterflooding. A comprehensive study was conducted based on all the petrophysical, seismic, geological, and reservoir engineering data. As a result, 3D geological and dynamic models were built. During the analysis, special focus was on geological structure and petrophysical characteristics as well as pressure transient analysis due to drastic changes in reservoir properties within small distances. In addition, the production history and the main stimulation, and EOR measurements, especially fracturing, acidizing and nitrogen huff ‘n’ puff results, were taken into account. Eventually, the waterflooding was simulated to identify its benefits and possible field application. Based on comprehensive study, the field is divided into several zones. The main criteria for dividing became a lithology that is represented by weathered crust, pure limestones, shaly limestones, limy claystones, and bioliths. Each part requires a different approach in development. If the north part is highly fractured and consists of limestones, water shut off is required due to active aquifer, middle part pressure support and the south part horizontal drilling or sidetracking with multistage high-tonnage fracturing. As a result, waterflooding was decided to be implemented in the part of the field characterized by ultralow permeability formation. Thus, recently, four producers were converted to water injection along with the fact that the injection is performed using low-salinity water. Surprisingly, injectors' rates are high enough, current voidage replacement ratio is sufficient and the results are unexpected. The reservoir is very complex where the lithology is drastically distributed within a small area. However, most of the territory is represented by very tight formation. Eventually, after several years of fields operation, many aspects became clear after the research work and development strategy was updated. So, the company is ready to develop such an unconventional reservoir that is one of the few fields in Kazakhstan.