No abstract

Pressure maintenance by gas injection in gas cap is one of the well-established methods for improving the ultimate recovery. Gas injection in the crestal part of reservoir into the primary or secondary gas cap for pressure maintenance is generally used in reservoirs with thick oil columns and good vertical permeability and this process is called gravity drainage. This paper comprises methodology and results of study to evaluate the feasibility of gas injection in gas cap for maintenance of reservoir pressure and to envisage incremental oil gain of a mature offshore carbonate field located in western offshore of India. Field has already produced more than 30% oil of its initial inplace volume. Water injection was started after 4 years of production and currently field is producing oil with 90% water cut. After one year of initial production phase the field producing GOR rose to two to three fold of its initial value mainly due to contribution of gas from gas cap. Depletion of gas cap gas made significant adverse impact on reservoir pressure and also fast pressure depletion from crestal part had allowed water breakthrough of injection and aquifer water to oil producers. At this stage to reduce the decline rate of wells for maximizing the future recovery without drilling of new wells and also without extension of existing infrastructure, the injection of gas in depleted small gas cap have been studied. In order to evaluate the feasibility of gas injection in depleted gas cap and its overall impact on oil recovery, three aspects were seen. First the optimized quantity of gas injection and its sensitivity along with the number of gas injectors were decided through reservoir simulation. Therefore, suboptimal oil producers falling within gas cap area are chosen for conversion to Gas injectors. Secondly injection gas requirement for the process will be fulfilled partly through the recycling of produced gas and rest from free gas production from another pay of the same field. Finally it is examined that current existing facility of gas compression will sufficiently cater the additional requirement of gas compression. The process will have additional 10 to 11% contribution in future oil production. The process of charging gas cap will provide additional support over ongoing water injection leading to a significant additional oil recovery by reducing the oil decline rate.

A successful water injection management is a key to increase or stabilize oil production and to maximize oil recovery from a mature field. This paper describes an approach to draw maximum benefit through existing set up of a water injection in a mature offshore carbonate field of India. Water injection initiated after the six years of oil production and field is under water flood since last 28 years. The field witnessed favorable water flood condition and almost negligible aquifer support. During its long production period most of the producers had been sideracked from one to three times to target better saturation areas which has led to uneven subsurface water distribution. The field has also suffered less voidage compensation for quite some time. To understand and mitigate the problem, a small pilot area within a field has been selected for implementing a good surveillance and monitoring program with pattern injection and possible intervention strategy. It was decided that based on the success of this pilot, the concept would be developed for implementation in step by step manner for entire field. The importance of multidisciplinary team has been recognized and detailed SWOT analysis was done for effective implementation of plan. Initially pilot area comprised of 15 oil producers and 4 water injectors. Conversion of one producer to water injector and restoration of water injection in 3 injectors were done as per plan and optimized injection rate (in this case maximum 3000 bbl per day) per injector were implemented. Peripheral pattern for pilot area with 5 injectors and 5 spot inverted patterns from rest 3 injectors were decided. After one year of the implementation a thorough performance analysis of the pilot has been carried out which indicates the overall improvement of liquid and oil production rates along with reduction in GOR and decreasing trend of oil decline rates of producers. The pilot approach has certainly helped to understand the Reservoir conformance in short duration of time. Encouraging results of this methodology guides to extent and implement this approach in other parts of field to cover the entire field in phased manner.

For planning the operations of Oil and Natural Gas Corporation Limited (ONGC) in the complex Heera field, it was estimated that over one hundred simulation runs would be needed to complete the history match of the field and almost the same number of simulations would be needed for production forecasting. Heera is a large field, with multiple faults and seven stacked carbonate formations. There are significant variations in petrophysical properties, and variable degrees of communication between reservoir zones. The simulation models include 479 wells with commingled production or injection. Well trajectories are complex and include multilateral and horizontal configurations. Field development options include use of simultaneous water alternating gas (SWAG) for enhanced oil recovery. Combining all these features, it would be difficult to run all the necessary sensitivity cases within the required project timeline, using a conventional reservoir simulator. Therefore, it was decided to test the applicability of a new generation simulation tool to address the challenges of the study. To ensure that the change of simulator would not impact the integrity of the model, rigorous quality checks were performed on the input data. After successful evaluation, the new software was used for the reservoir engineering study. The decision to apply the new simulator significantly reduced the elapsed time, with some realizations over 20 times faster compared to the original base case. As a result of this speed-up, numerous runs could be carried out to refine the history match. Multiple sensitivities could be used to help understand and reduce the uncertainties in a more comprehensive manner. Moreover, the prediction cases could be optimized to identify the best recovery strategy. This study has demonstrated the value of reducing simulation run times, to complete the project with greater efficiency and more confidence in the results. In future studies, high performance software tools can also enable use of fine resolution models, to capture detailed heterogeneities and optimize areal and vertical sweep.