Formation Evaluation Challenges in Thin Bedded Reservoirs: How to Effectively Test This Formation?

Abstract

Thin beds are a prominent feature in many recently discovered hydrocarbon reservoirs around the world. Not only are they typically difficult to evaluate, which leads to high reserves uncertainty, they are also frequently associated with inconsistent well performance. One of the main challenges in the evaluation of thinly-bedded reservoirs is an accurate productivity assessment. High-resolution borehole image logs provide us with detailed information on the internal structure of reservoirs sands, sand quality indications as well as a net-to-gross determination. Reliable formation evaluation requires further high-resolution petrophysical and geophysical logs (for an initial permeability estimation, free fluid volume, saturation, formation anisotropy and structural information). However, as per reservoir testing information (pressure, fluid types, PVT properties, permeability and producibility), this dynamic reservoir characterization method needs to be properly planned and evaluated. In the past, the use of full scale Drill Stem Test and Production Tests were conducted to obtain reservoir parameters including zone productivity. However, due to costly operations especially in marginal and also deepwater reservoirs, different scales of pressure transient test have been introduced1, 2, and 3. Several publications have discussed the use of smaller scale testing to obtain reservoir information. However, there have been no publications showing a complete comparison of pressure transient data obtained from several scales of measurement when heterogeneity is present in the reservoirs. This paper therefore aims to present a comparison of results obtained from alternative formation testing methods in a relatively thin (ten of meters) formation which are a single probe Wireline Formation Tester, dual packer Wireline Formation Tester, and full scale well test. This paper also discusses ways to optimise the testing methodology in such thin reservoirs. The application of an Interval Pressure Transient Test (IPTT) using a single probe as well as the dual packer WFT are illustrated and discussed in details. Their applications, specifications, advantages and disadvantages over the conventional testing method are extensively covered. This study aims to aid in devising suitable field development strategies at an acceptable cost while maintaining satisfactory operational efficiency.