A New Formation Tester and Its Applications in Extreme Ultrahigh Temperature Reservoirs

Abstract

As oil and gas exploration and production extends to deeper buried reservoirs, challenges such as lower porosities and Ultra High Temperature have been encountered. Several reservoirs in the Asian region, the North Malay basins in the joint development area between Thailand and Malaysia, and the Baiyun Sag and Qiong Dongnan basin in offshore China are considered to have the highest known temperature gradients due to their geological depositional system and hydrocarbon charging mechanism. More than fifty percent of wells drilled in these areas have temperature close to/or higher than 170 degC, and some reach above 200 degC. In number a of projects in these areas, the logging requires tools that can withstand up to 230 degC. Traditional, wireline Formation Testers (FT) with fixed rate and volume pre-test and old sampling technique using a dumping chamber (i.e. without pumping capability) had been the standard formation tester when temperatures reached 400degF (204 degC) and higher. The tools were not flasked and therefore, the temperature transient affected the quality and accuracy of pressure data1,2. Also, in such harsh environment, it is very difficult and time consuming to go back to a good mobility station for sampling after pressure measurement, due to reservoir heterogeneity and depth error. This paper discusses a project for a new slim hole ultrahigh temperature Wireline Formation Tester designed to obtain both pressure profiles and perform downhole Pressure Volume Temperature (PVT) *Trademark of Schlumberger fluid sampling with pump-out capability and downhole fluid sensors such as viscosity, density and resistivity in extreme HT environments. In addition, this slim hole ultrahigh temperature tool dimension has more clearance between the tool and formation, and therefore, less chance of having this tool get stuck during slim hole logging. The tool was first deployed in the North Malay Basin and since early 2018, new well head platform with five development wells were logged where a total of 76 pre-tests, four pump-out and ten fluid sampling stations were conducted. The main objectives for this FT tool were to obtain formation pressure, identify reservoir fluid and quantitative CO2 measurements zone by zone. The results will be discussed operationally and technically, in terms of data quality and accuracy and compared with on-site surface analysis. In addition, this tool improves significantly operationally compared to the previous tools and with some operators having mixed perceptions on running Wireline FT tool with bigger ODs, especially drilling departments, having this new slim hole with its smaller OD increases their confidence level in running it. For Deepwater Offshore China, an operator has been facing challenges to explore a brand-new block such as pore pressure distributions profile, reservoir quality, and extended logging period. The main objectives for the extreme FT are to obtain the formation pressure for drilling purpose, to understand reservoir potential to optimize the perforation interval for Drill Stem Test, and to narrow logging operation time window due to seasonal weather. This new ultra-high slim hole was therefore proposed to log in this challenging environment. This field example shows a significantly improved pre-test and sampling capability in the lower mobility ranges, which some previous generations of formation testers had struggled with in the past, in one run and without sacrificing testing efficiency The effective time for valid pretest can be achieved even in the range of mobility 0.01 mD/cp, high pressure of > 11000 psi, and high temperature of >180 degC. This paper discusses pre-job planning and actual job execution results in both locations. The challenges of logging and lesson learned are addressed. This is the first attempt in evaluating reservoirs in the deeper and HT sections to properly understand reservoir fluids.