Optimizing the Perforating Strategy in Well Completions To Maximize Productivity
- D.R. Underdown (Britannia Operator Ltd.) | W.H. Jenkins (Britannia Operator Ltd.) | A. Pitts (Britannia Operator Ltd.) | A. Venkitaraman (Schlumberger Reservoir Completions) | H. Li (Schlumberger Reservoir Completions)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2003
- Document Type
- Journal Paper
- 177 - 181
- 2003. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 4.6 Natural Gas, 5.2.1 Phase Behavior and PVT Measurements, 1.14 Casing and Cementing, 3.2.5 Produced Sand / Solids Management and Control, 2.4.3 Sand/Solids Control, 2 Well Completion, 2.7.1 Completion Fluids, 1.8 Formation Damage, 2.2.2 Perforating, 5.2 Reservoir Fluid Dynamics, 1.6 Drilling Operations
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The Britannia field is a gas-producing reservoir operated jointly by Chevron and Conoco. Before completion of the wells, the major concern was the optimum underbalance to obtain zero perforation skin during completion. Theoretical models were used to predict the optimum underbalance based on log-derived formation properties. With 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 at high underbalance pressures. Reservoir and outcrop core samples were perforated at simulated downhole 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). An underbalance value of 1,000 psi used in the outcrop sample tests indicated near-zero perforation skin. A perforation strategy for the North Sea field was chosen based on the results from this 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.
The major concern associated with completion in the Britannia field was the optimum underbalance required to achieve a zero perforation skin. In addition, an overall perforating strategy was desired because it 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 damage.1 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) with inflow/nodal analysis programs.2 For the current study, the variation in lithology was taken into account instead of the general procedure for average reservoir properties. Log and core data were used to determine the productivity of different layers based on their conductivity and formation damage. Numerical productivity simulations were carried out for each layer 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 perforating.3 Theoretical guidelines are available that can determine the optimum underbalance (zero perforation skin). The most frequently used optimum underbalance relationships are based on single-shot perforation and flow tests with oil-saturated samples.3 Very few tests or studies have been conducted to study perforation performance in gas-saturated core samples.4,5 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 and outcrop rocks simulating downhole 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 with the single-shot perforation and flow studies even though this was not a real concern in the completion because of the strength of the formation rock. The experiments were conducted in the Advanced Flow Laboratory in the Schlumberger Reservoir Completions Technology Center.
Available log and core data from a subject well were used to determine the optimum underbalance for zero perforation skin. Analysis of the log data and the rock mechanics program output for the interval showed a mean strength close to 10,000 psi unconfined compressive strength (UCS) and an average permeability of 20 md (with the exception of two high permeability streaks of 300 md). Based on these properties, the optimum underbalance pressure requirements were calculated (Fig. 1) with the following relationships.3
Equations 1 and 2
The calculations returned an average underbalance pressure requirement of 4,000 psi (with a lower range of 2,000 psi for the high permeability streaks and an upper range of 6,000 psi for the lower permeability streaks).
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