A case study on improving waterflood surveillance aided by a better understanding of the correlation between various water injectors and oil producers completed in the shallowest sub-layer of a giant multi-layered matured carbonate reservoir in Mumbai Offshore Basin is presented here. This understanding is then used to gauge effectiveness of the prolonged waterflood programme and to identify ‘target wells’ for optimizing water injection rate. The inferences of this analysis were tested using a simulation model.
Production, injection and pressure data of all wells completed in this sub-layer were extracted. The reservoir injection and withdrawal rates were computed using PVT data which were subsequently fed into an in-house developed streamline simulation program that generates a matrix of flow-based well rate allocation factors (WAF) correlating injection to withdrawal for each individual well as a part of its output. The analysis of injection efficiency per well was carried out in two scenarios viz. with current rates for effective waterflood surveillance and at a cumulative level with averaged rates to identify areas of deficiencies and optimize future injection rates.
Flow-based allocation factors provided a better picture than traditionally employed distance weighted technique owing to the underlying physics involved in describing streamline distribution in the reservoir. Results of analysis at the cumulative level indicated wells where injection efficiency, as measured by the ratio of injection rate to sum of streamlines-weighted withdrawal rates from connected producers, substantially deviates from 1. Few wells had an injector efficiency significantly higher than 1 which defined over-injection and potential recycling while a large number of injector wells had ratios of less than 1, highlighting the need to step-up injection rates and devise strategies for rigorous surveillance. To achieve the latter objective, injection-centric WAF's were regenerated at current situation with current rates and the dynamic nature of these factors could be observed by noting their slight difference with respect to previously estimated factors. This is attributed to averaged-out flow rates limiting the influence of newer high-rate producers and injectors. Nonetheless, wells in areas demanding attention are identified and requisite injection rates are assigned. These changes are included in the history-matched simulation model used for redevelopment activities and results were compared with a do-nothing case. The significant incremental recovery proves as a validation of the methodology adopted.
Waterflood surveillance on a well-to-well basis is always difficult in a matured field where water injectors are deployed in a ubiquitous fashion. This approach has rarely been employed in a reservoir of the size of Mumbai High and can be extended to other sub-layers subject to positive results from field implementation. Thus it is an endeavour to monitor waterflood effectiveness at a large field scale and could be beneficial for similarly developed fields.
The paper presents a case of applying classical reservoir engineering technique of material balance to one of the major carbonate reservoir in the western offshore basin in India that eventually led to establishment of more hydrocarbon volumes.
During Material Balance calculation, multiple runs were performed to match the pressure performance with a balance between the aquifer strength and hydrocarbon volume that was in agreement with geological understanding and performance of the field. The analysis indicated extra energy support that may be in the form of aquifer or higher in-place volumes. Following the in-house developed SIMEX (Simultaneous Exploration) approach a vertical well was identified for testing below assumed lowest known oil (LKO) limit.
The material balance study formed the basis for revisiting the geological understanding. The establishment of oil through the testing of well necessitated the revision of geological maps and re-estimation of hydrocarbon in-place volumes. Accordingly property maps have been prepared and volumes are revised. The revised volumes are about 14% more than the previous estimation. Similar approach was successfully applied to another reservoir in the Mumbai High field. Presence of more established oil will help in planning future strategies for field development.
Especially in fields where enough pressure production history is available, it is important to reassess the field's potential from time to time through simple and classical techniques available. Fields with multiple reservoirs have added advantage of developing the established hydrocarbons through zone transfer and in turn saving significant cost of drilling new well. This being a proven and classical technique, can be applied to other analogous reservoirs.
Kumar Singh, Raman (Oil & Natural Gas Corp. Ltd.) | Muralidharan, K. (Oil & Natural Gas Corp. Ltd.) | Rathore, Anil Kumar Singh (Oil & Natural Gas Corporation Ltd.) | Lohar, Babu Lal (Oil & Natural Gas Corp. Ltd.) | Verma, Shailendra Kumar (Oil and Natural Gas Corporation Ltd.) | Ram, Binay (Oil & Natural Gas Corporation Ltd.) | Patel, S.K. (Oil & Natural Gas Corporation Ltd.)
Understanding the complexity and characterizing a carbonate reservoir with complex heterogeneity is a daunting task that various operators of the E&P industry are facing today. Though the most perfect and quantitative answer of the complexity of dynamics of the carbonates comes from full field dynamic simulation, the simulation approach is often time consuming and expensive. A quick and less expensive approach is qualitatively looking at the petrophysical and production data in an integrated way for reservoir characterization and applying the findings in further field development.
Often in the brown fields, a large amount of petrophysical and dynamic data in terms of production and pressure are available. Especially in the brown fields under extensive and prolonged water flood, studying the varying level of interaction of producer-injector pairs in the field combined with the variation in the static properties can give very useful insight in the reservoir heterogeneity and identification of preferentially water flooded areas. This interpretation can be used to improve efficiency of water flood by adding and/or relocating injectors. The above concept of integrated static and dynamic data analysis has been applied for the multilayered carbonate reservoir of Mumbai High South field.
The integrated approach of static properties validated with production-pressure data helped in identifying the in-homogeneity, interlayer communication, nature of seismic picked faults and existence of flow barriers. Also the production-injection interaction analysis helped in identifying the likely pathways of water flood and the areas lacking water injection support. The findings have been successfully validated with subsequent pressure surveys. The above study under taken for the first time for Mumbai High South field has helped in improving the water injection plan with confirmation of the planned injectors and identification of areas needing more injection support.
Although non-predictive, the technique has immense value in validation of ongoing water injection scheme, providing understanding of inter-well communication and quickly bringing out areas requiring review of the injection plan for improving its effectiveness.
Mehsana tectonic block in North Cambay Basin is a fairly well explored, productive block where oil is both heavy and light. The block is now in a mature stage of exploitation associated with EOR techniques. Since oil production commenced in 1968-69 with a meagre 26 tons/day, the main fields have been developed through well planned development schemes. The recovery factor is intimately associated with reservoir heterogeneities, manifestation of environments of deposition, drive mechanism, fluid properties, pool size and depth of occurrence. These inherent characteristics are further influenced by reservoir management practices. The decline in oil production of the nineties was arrested and consistent growth since 1999 has been effected through efficient reservoir management practices accentuated by commencement of induction of technology.
IOR strategies with the objective of enhancing (i) Recovery and (ii) Production; are in vogue in the asset and began with revisiting the field geological and reservoir models. Strategies include:(a)Infill drilling recovery mechanism;(b)Initiation/Modification of water injection;(c) Enhanced oil recovery through Insitu combustion technique in heavy oil fields (d) High technology drilling like horizontal, high-angle-high drift wells and ultra-short radius drain holes and (e) By passed reserves identification and recovery methods like microbial methods addressing recovery enhancement. Production sustenance and enhancement through (a) Artificial Lift optimisation (b) Massive hydraulic fracturing (c) Well bore chemical stimulation in heavy oil fields and paraffin intervention in light oilfields(d) Flue gas utilization (e) Unique flue gas shut off in heavy oil belt (f) Improved sand control measures and (g) Improved sick well treatment are in place. The results from these schemes in terms of incremental oil are encouraging and have helped increase oil production marking a paradigm shift from earlier practices. Plans are afoot to foster improved production growth rate to increase ultimate recovery in tune with the newly set standards and accelerate brownfield development.