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.
This paper discusses case histories with solutions-oriented approach to shale gas cementation problems. Shale gas exploration and production around the globe in general, and US in particular has witnessed surge in activity to the extent that Shale gas is often regarded as ‘a game changer’ in the hydrocarbon industry. The success in United States has prompted the governments of several countries to develop their own shale gas programmes.
In India, estimates of shale gas resources vary from 63 Trillion Cubic Feet by Energy Information Administration (US). In order to tap this resource, Government of India came out with a Shale Gas and Oil Policy in 2013. Consequently, ONGC and OIL were given blocks mainly in Gujarat to test shale production technology and potential.
ONGC drilled 1st shale gas well # JMSGA of Jambusar field and well # GNSGA of Gandhar field, Cambay basin successfully. But CBL - VDL shown very poor results. After this failures, two more shale gas wells were awarded to service provider to carry out cementation for GNSGC and GNSGB wherein the cement slurries were used by adding proprietary chemicals. Results shown some improvement in CBL-VDL against shale gas.
Poor CBL-VDL creates a problem in production operation both for geothermal wells and oil wells. To accomplish good cement bond, complete removal of mud /all well fluid is necessary to enhance the displacement efficiency by maintaining desired rheological properties. However, in spite of this, many times the cement bond leading to poor bonding/ insufficient zonal isolation.
As a part of strategic planning it was decided to develop in house innovative expertise to solve the problems. Accordingly an extensive experimental analysis were carried out specially using expanding additives (3-5% bwow) in cement system, exhibited improved properties like Right Angle Set, Compressive strength, Stability, fluid loss control and desired rheology. Practically, combinations of salts in cements and spacers in pre flushes in cementing fluids have provided means for managing shale instability. Learning experience from past utmost care was taken for optimization of rheological hierarchy for mud, spacer and cement using fluid friction chart and cement software shown a phenomenon improvements in achieving desired parameters.
The field trials have been carried out successfully in three shale wells in India. 1st Well # TVAU of Cauvery and recorded excellent CBL-VDL (1-2 mV) against shale gas. 2nd Well # WDAU of Ahmedabad. Achieved excellent slurry/spacer parameters.3rd Well # ROBB of Agartala for 5-1/2" casing cementation resulted in better CBL-VDL (02-07mV) against shale zones.
These innovative techniques are cost effective, easy to handle and technically suitable. Also establishes in-house technological advancement in designing cement slurries and become a proven approach for cementing Shale gas wells in near future.
Prakash, S. (BG Exploration & Production India Limited) | Agrawal, V. (Schlumberger Middle East S.A.) | Govil, A. (Schlumberger Norge AS) | Bhide, M. (BG Exploration & Production India Limited) | Prakash, R. (BG Exploration & Production India Limited) | Kumar, Rajesh (Shell Technology India) | Mohanta, S. (Shell Technology India) | Kumar, Pradeep (ONGC Ltd)