Panna Formation is a very critical and challenging formation deposited during Paleocene time of geological past in various parts of Western Offshore Basin of India. It was deposited in a fluvio-deltaic environment, sometimes even in a restricted marine set-up. Such variation in depositional setting caused mineralogical complexity, which in-turn imposed a limitation in conventional approach of formation evaluation and saturation determination to identify the pay zones with confidence. A comprehensive approach of integrated formation evaluation for rock quality characterization was attempted using a combination of new generation elemental and acoustic analysis for delineating the potential hydrocarbon bearing zones independent of conventional resistivity-based approach along with a better insight on formation heterogeneity. The studied well was drilled up to Panna Formation and conventional open-hole logs were acquired while drilling. However, due to complex mineralogical nature of the formation, estimation of key critical reservoir parameters was very challenging and imposed higher uncertainties in the results. An alternate approach was adopted using a few advanced log measurements to address this challenge. A combination of new generation elemental and acoustic data has been recorded in a single wireline run after acquiring conventional basic logs while drilling. An accurate porosity was derived by eliminating various mineralogical assemblages along with estimation of a geochemical permeability based on detailed elemental analysis. Measured aluminum from neutron inelastic capture spectrum method enabled to estimate clay volumes with accuracy, which provided the required insight for better effective porosity in such shaly-sand environment. Based on this improved porosity and permeability, an approach for rock-quality indexing was used for reservoir delineation.
Moreover, a good amount of organic carbon was found associated with clays caused shales with higher resistivity. Based on elemental measurements an interesting insight was possible to extract for resistivity independent fluid saturation. An additional pay zone with hydrocarbon saturation based on such resistivity independent approach was possible to identify, which was masked by conventional resistivity-based interpretation. Acoustic analysis results assisted in delineating the zones with possible open fractures to avoid any possibility for unwanted fluid breakthrough.
Based on this approach of alternate integrated petrophysical analysis perforation zones were selected including an additional zone, which was masked based on conventional analysis. The well was started producing around 1,05,000 m3 gas with around 200 barrels of oil per day. This study showcased an alternate and efficient characterization approach for any such mineralogically challenging clastic formations.
Kumar, Rajeev (Schlumberger) | Zacharia, Joseph (Schlumberger) | Guo Yu, Dai (Schlumberger) | Singh, Amit Kumar (Schlumberger) | Talreja, Rahul (Schlumberger) | Bandyopadhyay, Atanu (Schlumberger) | Subbiah, Surej Kumar (Schlumberger)
The unconventional reservoirs have emerged as major hydrocarbon prospects and optimum yield from these reservoirs is dependent on two key aspects, viz. well design and hydrofracturing wherein rock mechanics inputs play key role. The Sonic Measurements at borehole condition are used to compute the rock mechanical properties like Stress profile, Young's Modulus and Poisson's Ratio. Often, these are influenced by the anisotropy of layers and variations in well deviation for same formations. In one of the fields under review, the sonic compressional slowness varied from 8us/ft. to 20us/ft. at the target depth in shale layer in different wells drilled with varying deviation through same formations. This affected the values of stress profile, Young's Modulus and Poisson's Ratio resulting in inaccurate hydro-fracture design. At higher well deviation, breakouts were frequently observed and could not be explained on the basis of compressional slowness as it suggested faster and more competent formation. Current paper showcases case studies where hole condition improved in new wells with better hydro fracturing jobs considering effect of anisotropy in Geomechanics workflow. Sonic logs in deviated wells across shale layer were verticalized using estimated Thomson parameters considering different well path through same layer and core test results. Vertical and horizontal Young's Modulus and Poisson's Ratio were estimated for shale layers with better accuracy. The horizontal tectonic strain was constrained using radial profiles of the three shear moduli obtained from the Stoneley and cross-dipole sonic logs at depth intervals where stress induced anisotropy can be observed in permeable sandstone layer. A rock mechanics model was prepared by history matching borehole failures, drilling events and hydro-frac results in vertical and horizontal wells using updated rock properties. Geomechanical model with corrected sonic data helped to explain the breakouts in shale layer at 60deg-85deg well deviation where the original sonic basic data suggested faster and more competent formation with slight variation in stress profile among shale-sand layer. Considering shear failure, the mud weight to maintain good hole conditions at 80deg should be 0.6ppg-0.8ppg higher than that being used in offset vertical wells. Estimated closure pressure and breakdown pressure showed good match with frac results in deviated wells using new workflow. There was difference of .03psi/ft-0.07psi/ft. in shale layers using this new workflow which helped to explain frac height and containment during pressure history match. This paper elucidates the methodology that provides a reliable and accurate rock mechanics characterization to be used for well engineering applications. The study facilitates in safely and successfully drilling wells with lesser drilling issues and optimized frac stages.
Reddy, S. S. (Oil and Natural Gas Ltd) | Anjaneyulu, J. V. (Oil and Natural Gas Ltd) | Lal, Abhay Kumar (Oil and Natural Gas Ltd) | Rao, E. J. (Oil and Natural Gas Ltd) | Ramakrishna, C. H. (Oil and Natural Gas Ltd) | Talreja, Rahul (Schlumberger) | Bahuguna, Somesh (Schlumberger) | Zacharia, Joseph (Schlumberger) | Chatterjee, Chandreyi (Schlumberger) | Basu, Jayanta (Schlumberger)
ONGC is working on fast-track development of Malleswaram field in Krishna-Godavari (KG) basin. Proven gas reserves are located in the in the late Cretaceous Nandigama formation which is located at a considerable depth. Exploration wells were drilled and overpressure of up to 1.8g/cc were encountered in Raghavapuram and Nandigama formations. Many drilling challenges related to overpressure were faced, including losses, tight hole, and stuck pipe. Apart from causing non-productive time (NPT), these events also increase the risk associated with drilling and completion of a well. Since, the development phase has just begun and geomechanics study was not incorporated in the exploration phase, there is enough scope to optimize drilling and reduce NPT. This study was conducted with an objective to optimize drilling and completion activities by integrating advanced acoustic and geochemical data for geomechanics based solution. This paper is focused on the new insight developed on the cause of overpressure and characterizing it. A detail analysis on stress direction and its variation with depth and among wells is also presented. In addition, wellbore stability models are also discussed to understand the requirement of appropriate drilling mud weights and casing policy in order to improve wellbore integrity, mitigate drilling risks, and ensure future drilling success. Minimum horizontal stress, breakdown gradient and rock elastic properties generated under the study are also used to generate completion quality flags to aid in identification of stress barriers within reservoir sand units of Nandigama formation.
Reddy, S. S. (Oil and Natural Gas Ltd) | Anjaneyulu, J. V. (Oil and Natural Gas Ltd) | Lal, Abhay Kumar (Oil and Natural Gas Ltd) | Rao, E. J. (Oil and Natural Gas Ltd) | C H, Ramakrishna (Oil and Natural Gas Ltd) | Talreja, Rahul (Schlumberger) | Bahuguna, Somesh (Schlumberger) | Zacharia, Joseph (Schlumberger) | Chatterjee, Chandreyi (Schlumberger) | Basu, Jayanta (Schlumberger)
Malleswaram field in Krishna-Godavari (KG) basin has proven gas reserves in the late Cretaceous Nandigama formation. Many drilling challenges were faced, including losses, tight hole, and stuck pipe in the Raghavapuram and Nandigama formations overlying the reservoir interval. This study was conducted to provide a solution for drilling optimization by mitigating drilling-related nonproductive time (NPT). Integration of acoustic and geochemical data for geomechanics study provided a new insight into cause of overpressure and need for revamping of casing policy to significantly improve wellbore stability, mitigate risks, and ensure future drilling success. Generated stress models can be used to optimize hydraulic fracturing in these reservoirs. A completion quality based on stress model indicates the need for multistage fracturing due to the presence of stress barriers inside sand units in Nandigama formation.
Roy, Sunit (Oil and Natural Gas Corporation Ltd.) | Roy, G. K. S (Oil and Natural Gas Corporation Ltd.) | Chauhan, R. S. (Oil and Natural Gas Corporation Ltd.) | Bahuguna, Somesh (Schlumberger Asia Services Ltd.) | Zacharia, Joseph (Schlumberger Asia Services Ltd.) | Chatterjee, Chandreyi (Schlumberger Asia Services Ltd.) | Basu, Jayanta (Schlumberger Asia Services Ltd.) | Bhuyan, Priyanuz (Schlumberger Asia Services Ltd.) | Talreja, Rahul (Schlumberger Asia Services Ltd.)
In recent years, exploration in northeast India has focused mainly in the area south of the Naga fault. This area is tectonically active with complex folding and faulting. Target formations are in excess of 3,000m through these complex structures. ONGC has drilled several wells in the Tripura region of northeast India. Although results were encouraging, the main challenges faced during drilling were various well control and instability events: kicks, tight hole, stuck pipe, lost in hole, etc. The causes of this instability were difficult to isolate. Experience from past wells also indicated the existence of overpressure in the Middle Bhuban Formation, which could not be accurately predicted by surface seismic data. Hence, drilling through these highly stressed and overpressured formations led to excessive NPT and cost. In addition, severe hole enlargement and rugosity through eventual reservoir sections resulted in poor logging conditions and uncertain reservoir evaluation. As a result, access and interpretation of the reservoirs, to prove and produce reserves, is a major challenge in this region. In this paper, two case studies are presented to show how a geomechanics-based approach has significantly improved drilling rates by reducing the drilling-related problems. New wells were drilled within 40% of the planned number of days. Better hole conditions not only improved drilling performance but also led to huge improvements in reservoir evaluation and the identification of unloading mechanisms, probably caused by uplift due to thrusting, in the Middle Bhuban Formation. Integrated study provided valuable information about overpressure and stresses acting in this field/structure for future drilling.