Optimizing Perforating Strategy in Well Completions to Maximize Productivity

Abstract

The Britannia field is a gas producing reservoir operated jointly by Chevron and Conoco. Prior to completion of the wells, two major concerns were raised: potential sand production (perforation stability), and the optimum underbalance (for zero perforation skin) during completion. Theoretical models were used to predict the optimum underbalance based on log derived formation properties. Using detailed log permeability data numerous simulations were carried out to choose the gun, charges, shot density and perforating strategy for optimum productivity. Different shot densities and charges were used in different sections of the formation based on simulation results as opposed to choosing perforation parameters based on average well properties. Experiments were conducted to confirm the theoretical underbalance predictions and to investigate the stability of perforation tunnels at high underbalance pressures. Reservoir and outcrop core samples were perforated at simulated down hole conditions in the laboratory at different underbalances based on values obtained from the model. Flow performance evaluation of the perforated reservoir core samples confirmed earlier conclusions on the sensitivity of the formation to aqueous wellbore fluids (brine). The results also confirmed the stability of the perforation tunnels at high underbalance. An underbalance value of 1000 psi used in the outcrop sample tests indicated near zero perforation skin. Perforation strategy for the North Sea field was chosen based on the results from the study. Well performance analyses of 12 of the wells completed indicate low to negative skins. The information is presented in this paper as a case of how to design a good perforating job and also to emphasize the need to study optimum underbalance for gas formations. Introduction The Britannia field is a gas-producing reservoir operated jointly by Chevron and Conoco. Prior to completion of the wells, two major concerns were raised: potential sand production, and the optimum underbalance (for zero perforation skin) during completion. In addition an overall perforating strategy was desired as this has a major impact on well productivity. In general, there are four key aspects to perforating that play an important role in determining the productivity: perforation dimensions (length and diameter), shot density, phasing and perforation damage1. The choice of gun parameters to optimize the completion is usually carried out using theoretical analysis of the efficiency of the completion (or gun choice) using inflow/nodal analysis programs2. For the current study the variation in lithology was taken into account instead of the general procedure of average reservoir properties. Log and core data was used to determine the productivity of different layers based on their conductivity and formation damage. For each layer numerical productivity simulations were carried out to determine the optimum perforation parameters: shot density, penetration and underbalance conditions (an acceptable phasing was fixed). The best method to minimize perforation damage is by underbalance perforating3. Theoretical guidelines are available which can determine optimum underbalance (zero perforation skin). The most frequently used optimum underbalance relationships are based on single-shot perforation and flow tests with oil saturated samples3. Very few tests or studies have been conducted to study perforation performance in gas saturated core samples4,5. In addition use of the theoretical models can lead to high underbalance pressure requirements in strong low permeability formations. This issue was addressed in this study by single-shot perforation and flow experiments in reservoir rock and outcrop rock simulating down hole stress and flow conditions. One of the concerns during underbalance perforating is the potential for sand production (or collapse of perforation tunnels)6. This was also addressed using the single-shot perforation and flow studies. The experiments were conducted in the Advanced Flow Laboratory in Schlumberger Reservoir Completions Technology Center.

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