The Characterization of Geological Flow Units in Carbonate Reservoirs - Integration of Core, Log & Production Logging Data

Abstract

Geological flow units based on reservoir rock typing can be used to characterize geological facies based on their dynamic behavior. A procedure for identifying and characterizing geological flow units helps resolve some of the key challenges faced in exploration and production of carbonate reservoirs. The current work focuses on the rock typing/flow units classification for reservoir characterization in a carbonate gas reservoir. The Sui Main Limestone (SML) Eocene reservoir, located in the southwestern part of the Middle Indus Basin in Baluchistan Province was selected, and the study carried out using core, wireline logs of 30+ wells and production logging data. The reservoir was previously categorized based on the geological information into four layers.

An integrated workflow was used for characterization of Geological Flow Units based on all available information. The Sui Main Limestone (SML) reservoir was divided into three flow units/rock types, depending on the well log patterns, core data, Porosity–Water saturation relationships, flow zone indicators (FZIs), and the Porosity–Permeability relationships. Production logging data was used to validate flow unit classification.

The field is a gas bearing reservoir composed of medium hard to hard limestone characterized by high porosity but poor/low permeability. The matrix permeabilities and porosity range from 0.01–40 mD and 0–35 percent respectively. Three rock types and groups have been identified in the SML formation; the first group represents the poor reservoir quality (GFU-1) Highly cemented limestone, the second group reflects the moderate reservoir quality (GFU-2) Moderately cemented limestone, and the third group represents the very good reservoir quality (GFU-3) poorly cemented limestone.

Production logging data was used to validate flow unit classification. Pressure Transient Analysis (PTA) indicated that only partial penetration occurred, and therefore only a portion of the perforated area was contributing to production. Specifically, the majority of inflow zones identified in the production logging data corresponded to GFU-3 flow units. As expected, the other two flow units only provide a small increase in production. This further validated the partial penetration model determined from PTA.

Geological flow units/rock types in these thick carbonate reservoirs and can be helpful in selecting optimum perforation intervals rather than perforating the whole thickness. This innovative method can save significant CAPEX without compromising the production.