Identifying Layer Permeabilities and Skin Using a Multi-Layer Transient Testing Approach in a Complex Reservoir Environment

Conventional pressure transient testing, using a pressure gauge positioned at a fixed depth in a well, has historically been the main source of permeability and skin estimation in formations. However, if a well is completed as a multi-layer commingled producer, then this conventional approach makes it difficult to measure the permeability and skin of individual layers. Greater Munga field of the Greater Nile Petroleum Operating Company (GNPOC) in Sudan has several wells that commingle production from the Aradabia, Bentiu-2 and Bentiu-3 formations. These formations are highly variable in terms of the reservoir properties, oil types and pressure regimes. A selective inflow performance (SIP) test was carried out during production logging (PL) jobs in some of these wells and it indicated that the productivity index (P.I.) of the individual layers varies widely, ranging from 1.5 to 15 b/d/psi. This illustrated the need for a method to estimate the permeability and skin of each layer. This information was needed for reservoir model calibration, well productivity prediction, low productivity diagnosis and remedial action selection. Two solutions were proposed to GNPOC; use the conventional technique of isolating each layer and testing it separately or carry out a commingled multi-layer transient (MLT) test with a PL tool. In an MLT test, in addition to the normal PL runs, individual pressure transient stations are also recorded at the top of each contributing layer. The MLT test measures the flow rate and wellbore pressure above each producing layer for different surface flow rates during the infinite-acting phase. These individual layer flow rates and pressure transients are used to calculate the individual layer properties. GNPOC decided to go in for the MLT testing option and two wells were analyzed. In the first well, MLT testing showed that one of the layers had a very high permeability compared to the other layers. It depleted much faster and had early water breakthrough. Consequently a water shut-off job is planned for this layer. In the second well, MLT analysis showed that the upper layer had poorer permeability as compared to the lower layers. However, this layer holds good oil reserves. Hence, this well is a good candidate for future side tracking into the upper layer, in order to exploit the untapped reserves in this layer. In this paper, we will discuss the MLT testing technique, introduce a workflow for the analysis, and then will discuss the results of the analyses for two examples from GNPOC. Based on the success of these cases, multi-layer transient testing is estabilished as a preferred testing technique in this complex reservoir environment.