Stimulation Efforts in Carbonate Gas Reservoir Experiencing Subsidence in Offshore North West Java Field - Indonesia

Abstract

Subsidence and reservoir compaction continues to be significant concern for the oil and gas industry. The decrease of pore pressure during hydrocarbon production (depletion) leads to compaction of the reservoir. When compaction occurs, it changes the porosity and permeability properties of the reservoir rock and can affect recovery efficiency and well productivity. The results of stimulation efforts to improve gas deliverability in carbonate reservoir experiencing subsidence in Offshore North West Java (ONWJ) field are presented here. The wells in this study were completed in the Parigi Formation, which is a thick carbonate buildup formation with large columns of gas about 250–300 feet thick underlain by aquifer. It has high porosities (32–45%) and good gas permeabilities up to several Darcies with the initial well production were around 20–30 MMSCFD. The continuous production of gas from this shallow, thick and low strength carbonate reservoir has resulted in reservoir compaction, surface subsidence, several well mechanical failures and a sharp production decline (from 15 MMCFD to 2 MMCF). The other impact of declining gas rate that is nearing its critical velocity is causing water condensation that cannot be lifted out of the well; it then accumulates in the wellbore and creates more back pressure on the reservoir. Treatment efforts, starting from nitrogen kick-off with CT to unload the condense water in the wellbore followed by re-perforation and acidizing job, that were conducted in the first two well have resulted a significant production response. Gas production increases by 4–9 times after the treatment. The previous fear/perception that acidizing would perhaps even worsen the conditions of the reservoir and thus reduce production proved not to be true. These successful treatments have led to stimulation campaigns for other wells in the area. This paper will also discuss several surveillance projects to monitor and assess the magnitude and progression of surface subsidence, reservoir compaction and wellbore damage in this area. Introduction The Offshore North West Java (ONWJ) contract area was the first of BP's (formerly ARCO) concession areas in Indonesia. The area covers 6.8 million acres Offshore Northwest Java Island with water depth up to 50 meters, see figure 1. Production started from this area in late 1971 and was the first Indonesia offshore production. Currently, the field is producing around 45,000 BOPD and 250 MMCFD of gas sales. Starting in 1980's, BP (formerly ARCO) discovered a lot of gas potential in this area while exploring for oil. Major gas discoveries in this area were from Pre-Parigi, Parigi, Talang Akar and Main-Massive formations, which contain potential reserves up to 2 TCF of natural gas. The reserves are spread over 20 fields. Two of the main gas contributing areas are the KLX field & the adjacent KLY field, both of which were developed in 1993. The fields were developed into four production platforms (KLXA, KLXB, KLYA, and KLYB platforms) and consist of a total of 10 wells. The wells are typically completed in 9- 5/8" casing with 7" and/or 5–1/2" production string. These fields have excellent deliverability and constitute the backbone of the ONWJ gas supply program. These fields produce from the Middle Miocene Parigi formation at a depth of about 1,900 feet. Large columns of gas (about 250–300 feet thick) underlain by aquifers characterize the KLX and KLY reservoirs. The KLX reservoir is underlain by a 200 ft thick aquifer, while the KLY reservoir is filled to the spill-point and the aquifer is only 40 feet thick in certain areas. The fraction of gas in the total carbonate reservoir column is about 0.5 for KLX and 0.9 for KLY (1). The continuous production of gas from this shallow, thick and low strength carbonate reservoir has resulted in reservoir compaction, surface subsidence, several well/facility failures, and sharp production decline. The other impact of declining gas rates that are nearing critical velocity are causing liquid/water condensation, which then accumulates in the wellbore. It could not be lifted out and created more back pressure on the reservoir. This condensation water could be lifted out continuously if the production rate were above its critical velocity, ±2 to 4 MMCFD (based on M. Li et al. 15).