Challenges on Sampling and Characterization of Heavy Oil in Deep Exploratory Wells: A Field Case Study

Abstract

Abstract The objective of this paper is to present the methodology adopted to overcome challenges faced during sampling and characterization of heavy oil in deep exploratory wells. As a part of exploration activities, two exploratory wells had been successfully drilled and tested in deeper low permeability Lower Cretaceous reservoirs. Drill Stem Test (DST) technique is adopted to test all exploratory wells for collecting production, surface and bottom-hole pressure and temperature data and collection of fluid samples for fluid characterization. As the wells are tested with retrievable packer, coiled tubing (CT) is required during activation and stimulation operations due to absence of other lifting methods. In this case study wells, coiled tubing was lowered and the well was lifted continuously with nitrogen for clean-up and to assess production behavior as there was no self-flow. Surface samples have been checked to assess contamination with diesel used for underbalance operations. Continuous nitrogen lifting helped to collect representative bottom-hole samples. During testing, there was poor inflow of heavy oil of API 21–22 Deg, but the well could not be produced naturally in spite of repeated attempts. Matrix acid stimulation treatment was done to improve the productivity of the reservoir. Well flowed heavy oil and then gradually ceased to flow. Due to low mobility, self-flow could not be sustained. These pose significant challenges for well cleanup, flow studies and collection of representative sample for PVT analysis and fluid characterization. Surface sampling was not favorable due to mixing with lifting nitrogen gases and only alternative method available was to collect Bottom-Hole Samples (BHS). However, proper cleanup was an issue for sampling operation. Continuous lifting with nitrogen helped to produce clean oil in both the wells. Compositional analysis was done on the nitrogen lifted sample to quantify the contamination level before lowering the down-hole sampler in order to capture representative BHS. As a result of this approach, representative samples could be captured and fluid characterization could be carried out with quality results. It has been planned to adopt the same methods in such type of situation in testing exploratory wells. With the systematic approach representative samples could be successfully captured in both the wells where self-flow could not be sustained in spite of repeated attempts. Fluid characterization of heavy oil could be could be carried out in these two exploratory wells which are very valuable for further development planning of the reservoir / field.