A Change in Formation Evaluation Strategy to Enable Making Key Investment Decisions Faster

Abstract

An extensive exploration and appraisal campaign was conducted over the last four years in the East Coast of Africa. From the numerous wells drilled and evaluated, the formation evaluation program included a wireline formation tester equipped with a standard, large faced or elongated packer to conduct the extensive pressure testing and sampling program. In the initial phases of data acquisition the role of fluid sampling via wireline testing was primarily to confirm mobile fluid type leaving fluid samples for PVT analysis to be collected during flow testing of the wells. In the later stages of the program, a change of strategy was necessary to gain wireline formation tester samples of sufficiently high purity and quality for PVT testing. This was primarily due to there being insufficient time between drilling wells and making key investment decisions to allow well testing samples to be acquired. Through extensive experience, it has been observed that pressure profiles are not always conclusive to determine fluid unit content and contacts alone. Sampling and fluid ID pump outs were used extensively to help determine the mobile fluid phase and reserve extent. It was also concluded that at a certain mobility cut off, getting any sample was near impossible due to the viscosity and drawdown limitation. When conducting sampling, the colouration and properties of the OBM made trend evaluation and the contamination determination extremely difficult. On the spectroscopy front, the optical density trends were often on the most part linear, and the GOR evaluation was difficult as the acoustic fluid response at the reservoir pressure and temperature was below that of the OBM. The density measurement followed the same linear trend, with only the viscosity and refractive index at times giving any meaningful trend to establish potential cut off periods on the pump – out. From the PVT analysis, it was obvious that no matter how long the pump- out and how flat the sensor responses were, contamination was never better then 12 – 15%. The service provider was challenged to overcome these obstacles and capture clean samples and pump out zones with low mobility to evaluate the extent of the reserves. The ability to flow the hydrocarbons at reasonable rates in the very low mobility units enabled the operator to prove and extend the reserves calculation of the wells, which would have been near impossible with the standard equipment.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018