Engineered Solution Helps Tackle Tricky Shale Gas Cementing Challenge in HP/HT to Deliver Good Cement Bond Across Shale Section Located in Western India

In a world where energy is a major concern, the revolution of shale gas globally has triggered a potential shift in thinking about production and consumption that no one would have expected. The enormous shale gas resources identified today are becoming game changers in many developing countries. The booming economy of India is seeing a significant increase in its energy demand, with industries establishing new footprints in the western region of the country. Operators are venturing into deeper and harsher conditions (HP/HT environments) to tap those resources. Even though shale gas is now found globally, it is still described as an unconventional source of hydrocarbons. This is because the extraction of shale gas is tricky and challenging. To unlock the unconventional gas reservoir most of the wells are horizontally drilled and hydraulically fractured. This process has a strong impact on cement bonding across the section. Firstly, the cement needs to provide an effective barrier in the annulus around the casing, which has been horizontally placed. Secondly, cement has to withstand various mechanical loads during hydraulic fracturing and ultimately over the life of the well. The present study covers the Navagam field located in the Ahmedabad block of North Cambay Basin. Cambay Basin is bounded on its eastern and western sides by basin-margin faults and extends south into the offshore Gulf of Cambay, limiting its onshore area to 7,900 mi2. The operator's western asset had already deployed its resources on evaluating the data to assess the potential shale gas in the Navagam block in the Cambay Basin. This paper highlights successful cement placement in an unconventional shale gas reservoir in onshore western India. It was crucial to understand why early exploration wells in the area resulted in poor initial zonal isolation in order to refine the asset development model for future wells. Based on these models, a mechanically modified resilient cement system was engineered. Subsequent exploration wells were then cemented with the resilient cement system to allow for dependable zonal isolation of reservoir bands permitting the accurate determination of discrete reservoir geomechanical properties within the overall reservoir target.

Interpreter's Corner

Basaltic reservoirs have produced hydrocarbon from Yurihara Field in Japan, Quiko Depression in China, and Padra Field of Cambay Basin and Western Offshore Basin in India. The availability of fractured, altered, and vesicular basalts contributes to reservoir development in this stratigraphic unit. This study is conducted in the Kutch-Saurashtra Basin, located at the western continental margin of the Indian subcontinent wherein, the Deccan basalt, with a thickness range of 200–2500 m, overlies Mesozoic sediments. The Jurassic and Cretaceous sediments constitute the main source rock in the area. Several wells have been drilled through the entire basalt section, and some are hydrocarbon bearing in basalt. The entire basalt section in the study area has been classified into four major units using gamma-ray logs. These units have been further subdivided into individual flows and correlated all over the basin. Analysis shows that the base of an individual basalt flow is massive, and the top is differentially altered. Crossplot analysis of P-impedance and VP/VS ratio carried out on logs delineates a zone of moderately weathered/altered basalt, which is due to spheroidal weathering and calibrated with sidewall cores. These moderately altered zones between two successive flows of basalt are the probable reservoir facies for hydrocarbon accumulation, provided that there is an overlying seal in the form of massive or completely altered basalt. Three-dimensional seismic data in the area show an alternating reflection pattern in the basalt section due to the alternation of massive and weathered basalt. The seismic signature of basalt in the area is similar to that of a sedimentary sequence in any given area. Continuity of the identified individual flows in seismic scale has been propagated over the entire seismic, and subsequent inversion has facilitated the deciphering of the probable hydrocarbon-bearing locales within basalt.

Depth and composition dependent nanopore structures of Indian shale gas reservoirs: An implication on storage potential

<p>We investigated the nanopore structures of shale samples obtained from Cambay and Krishna-Godavari (KG) basins in India using low-pressure N<sub>2</sub> sorption method. The samples occurred at variable depths (1403-2574m and 2599-2987m for Cambay and KG basins, respectively) and have wide ranges of clay contents (56-90%) both in volume and mineralogy. The results of this study indicate the specific surface area (SSA) and pore diameters of the samples share a non-linear negative correlation. The SSA is a strong function of the clay content over the samples’ depth. The specific micropore volumes of the KG basin have relatively higher (8.29-24.4%) than the Cambay basin (0.1-3.6%), which leads to higher SSA in the KG basin. From different statistical thickness equations, the Harkins Jura equation was found to be most suitable for the computation of BJH pore size distribution and t-plot inversion in shale. Shale samples from Cambay basin show unimodal pore size distribution, with a modal diameter of 4-5nm, while in the KG basin, show bi-modal to multimodal pore size distribution, mostly ranges from 3-12 nm. In the fractal FHH method, fractal exponent D<sub>f</sub>-3 provides a better realistic result than fractal dimensions calculated from (D<sub>f</sub>-3)/3. In our samples, pore surface fractal dimension (D<sub>f1</sub>) show a positive correlation with SSA and a negative correlation with pore diameter, and pore structure fractal dimension (D<sub>f2</sub>) shows a negative correlation both with clay(%) and depth. The experimental data obtained in this study are instrumental in developing the pore-network model to assess the hydrocarbon reserve and recovery in shale.</p>