Establishing and Enhancing Reservoir Injectivity in the B8/32 asset, Gulf of Thailand

Abstract

B8/32 is an oil and gas asset located offshore in the Gulf of Thailand. The asset consists of two main operating areas known as Benchamas and Tantawan fields. These fields are divided into several sub-operating areas where the wellhead platforms have been installed e.g. BEWA, BEWB, TAWA, TAWB, etc. Waterflooding has been in operation since late 2000 and has continued to increase with the increase of proven reservoir connectivity. In the Benchamas and Tantawan oil fields, establishing water injection has always been challenging. As a rule of thumb, most reservoirs initially take little or no water at the existing system injection pressure of +/-1200 psi. This applies to zones of varying permeabilities, pore pressures, and hydrocarbon saturations. Thus, with more and more waterfloods being implemented, the need to stimulate wells to achieve satisfactory injectivity has increased significantly over the past few years. Faced with a variety of challenges ranging from reservoir to operational aspects, both hydro-fracturing and acid stimulation approaches have been carried out with both successful and unsuccessful results. For successful stimulation jobs, average pre-stimulation injection rates range from 0 to 600 bpd while post-stimulation rates vary from 2,000 to 10,000 bpd. Utilizing the data from number of injectors, the study has been divided into three main parts; 1) Fracture initiation/reservoir opening pressure: based on many step rate tests conducted as part of the stimulation treatments, a study has been conducted to gain better idea about the range of the reservoir opening pressure at various reservoirs at different reservoir depths. This data set benefits other assets in the Gulf of Thailand where new water disposal or waterflood systems are being designed.2) Experiences with different stimulation techniques and how they have evolved over the past few years: two stimulation techniques which have become the regular methods employed to establish injectivity for B8/32's injectors are to be discussed. They are matrix acidizing and prolonged hydro-fracturing treatments. Recommendations are provided incorporating pros and cons of each technique.3) Predicting long term injectivity: practice has found discrepancies between theoretical and actual injectivity indices. The actual numbers are typically lower than those theoretically calculated, this can be partially attributed to uncertainties in formation parameters and fluid characteristics (e.g. water quality). Anticipating the achievable/actual rate in a variety of formations is very important. An empirical-mathematical model has been established for the purposes of ranking waterflood target, estimating the needs for stimulation for projects in the Gulf of Thailand.