Nimisha Vedanti

Seismic inversion tracks in situ combustion: A case study from Balol oil field, India

In situ combustion is one of the approaches used for secondary recovery of heavy oil in which time-lapse seismic data might be used to track the production process. We have analyzed three time-lapse seismic surveys carried out at a time interval of one year during pre- and postcombustion phases of in situ combustion at the Balol field in India. Interpretation of these land time-lapse data using standard seismic amplitude differences in terms of reservoir changes during production and injection can be erroneous. The Balol data, in particular, lacked calibration and had poor repeatability. We have addressed these issues by demonstrating the applicability of independent prestack inversion of baseline and monitor surveys to derive elastic attributes that help to produce clearer images of fluid movement. Independent estimates of wavelets from the baseline and two monitor surveys were used in prestack inversion to estimate acoustic impedance, shear impedance, and Poissons ratio. The results show unexpected movement of the thermal front away from the injection wells toward the north and northwest of the injection wells, farther from the production wells. These results have been confirmed by well production data from the field north and northwest of Balol.

Seismic monitoring of in situ combustion process in a heavy oil field

Three time-lapse 3D seismic surveys are analysed to monitor the effect of in situ combustion, a thermal-enhanced oil recovery process in the Balol heavy oil reservoir in India. The baseline data were acquired prior to the start of the in situ combustion process in four injection wells, while the two monitor surveys were acquired 1 and 2 years after injection start, respectively. We present the results of baseline and second monitor surveys. Fluid substitution studies based on acoustic well logs predict a seismic amplitude decrease at the top reservoir and an increase at the base reservoir. Both the amplitude dimming at the top reservoir and the brightening at the base reservoir are observed in the field data. The extent of the most pronounced 4D anomaly is estimated from the seismic amplitude and time shift analysis. The interesting result of seismic analysis is that the anomalies are laterally shifted towards the northwest, rather than the expected east, from the injector location suggesting a northwest movement of the in situ combustion front. No clear evidence of air leakage into other sand layers, neither above nor below the reservoir sand, is observed. This does not necessarily mean that all the injected air is following the reservoir sand, especially if the thief sand layers are thin. These layers might be difficult to observe on seismic data.

An efficient 1D OCCAM'S inversion algorithm using analytically computed first- and second-order derivatives for dc resistivity soundings

An efficient algorithm has been developed for 1D resistivity inversion problem using both first- and second-order derivatives, which are computed analytically. The second-order derivative matrix, which is not used in the OCCAMs inversion, has been incorporated into the algorithm employing analytical expressions. Computation of complicated second-order derivatives in each iteration is circumvented by a new algorithm. These modifications result in stable convergence of the OCCAMs inversion and in general, better misfit can be achieved specially for smoothing parameter, @m<1. The modified inversion algorithm, coded in MATLAB was tested using two synthetic Schlumberger resistivity sounding examples. Its application has been illustrated with field data from south India.

Was Archean Dharwar Craton ever stable? A seismic perspective

Indian shield comprising a number of Archean-Proterozoic cratons, forms one of the most dynamic, sheared and deformed continental segment amongst all stable areas of the earth. However, for a long time it has been believed that its lithosphere remained unaffected by episodic tectono-thermal and volcanic perturbations. Using available seismic data, an attempt has been made to study the seismic structure of the south Indian shield in order to examine the effects of such mantle processes on its overall crust-mantle structure. Our study suggests that by and large, Indian crust is intermediate to mafic in composition. On an average, only a couple of kilometers of the initially formed upper (granitic-gneissic) crust now remains in place and a thick chunk of the original lower crust has been consumed by the underlying thermally buoyant mantle due to sub-crustal erosion and then subsequently replaced by possibly as much as about 16 km thick magma layer on either side of the Moho. This study throws a new light on the geologic and geodynamic evolution of this region and contradicts the hitherto believed paleo-stability of the Indian shield.

Study of CO2 EOR in a Sector Model from Mature Oil Field, Cambay Basin, India

Among various Enhanced Oil Recovery (EOR) methods, gas injection has been proven to be one of the effective ways of enhancing oil recovery from mature fields. The field under study has approached the economic limit of production under conventional recovery methods (primary and secondary recovery). Since start of production in sixties, the field has produced 48.5 % of the initial oil in place and the water cut has increased to 89 % in April 2011. Responding to the industry needs, initially a comprehensive study was performed to evaluate the potential of immiscible CO2 injection for the recovery of residual oil after water flooding in this mature field. This paper presents the preliminary results of immiscible CO2 injection on the basis of laboratory studies and detailed compositional simulations carried out on a sector model of the field. Based on the results obtained from laboratory studies it was found that CO2 injection yields significant incremental recovery. Simulation results show significant increase in field oil production, essentially from 200 to 1100 m3/day and considerable decrease in water cut were observed. In addition, detailed PVT simulations were carried out to obtain an equation of state (EOS) that would better describe the phase changes in the reservoir. These results would form the basis for carrying out CO2 EOR simulations on a field scale.

Assessing the feasibility of CO 2 -enhanced oil recovery and storage in mature oil field: A case study from Cambay basin

The utilization of anthropogenic CO2 for enhanced oil recovery (EOR) can significantly extend the production life of an oil field, and help in the reduction of atmospheric emission of anthropogenic CO2 if sequestration is considered. This work summarizes the prospect of EOR and sequestration using CO2 flooding from an Indian mature oil field at Cambay basin through numerical modelling, simulation and pressure study based on limited data provided by the operator. To get an insight into CO2-EOR and safe storage process in this oil field, a conceptual sector model is developed and screening standard is proposed keeping in mind the major pay zone of the producing reservoir. To construct the geomodel, depth maps, well positions and coordinates, well data and well logs, perforation depths and distribution of petrophysical properties as well as fluid properties provided by the operator, has been considered. Based on the results from the present study, we identified that the reservoir has the potential for safe and economic geological sequestration of 15.04×106 metric ton CO2 in conjunction with a substantial increase in oil recovery of 10.4% of original oil in place. CO2-EOR and storage in this mature field has a bright application prospect since the findings of the present work could be a better input to manage the reservoir productivity, and the pressure field for significant enhancement of oil recovery followed by safe storage.

Seismic Monitoring of In-situ Combustion In the Balol Heavy Oil Field

Two 3D seismic surveys are analyzed to monitor production changes within the reservoir. The base data set was acquired prior to start of in-situ combustion process in four injection wells while the monitor survey was acquired 2 years later. Fluid substitution studies, based on acoustic well logs, predicts a seismic amplitude decrease at top reservoir and an increase at the base reservoir and a relative velocity slow down of up to 8%. Both the amplitude dimming at top reservoir and the brightening at the base reservoir are observed in the field data. However, the extent of the most pronounced 4D anomaly (50m) is lower than reservoir engineering expectation (100m). This difference is probably caused by the lateral shift between the gas zone and the hot zone within the combustion front. The anomalies are laterally shifted towards northwest of the injector location, which might be explained by the fact that the reservoir sands are dipping upwards in this direction. No clear evidence of air leakage into other sand layers, neither above nor below the reservoir sand, is observed. This does not necessarily mean that all the injected air is following the reservoir sand, especially if the thief sand layers are thin, which might be difficult to observe on seismic data.

A regularized Wiener–Hopf filter for inverting models with magnetic susceptibility

ABSTRACT For a magnetic target, the spatial magnetic signal can be expressed as a convolutional integral over Greens function of an assumed model with susceptibility as its parameter. A filter can be used to obtain the susceptibility by minimizing the mismatch between observed and the computed magnetic anomalies. In this perspective, we report the development of an advanced digital filter, which is efficient and can be used to map rock susceptibility from the acquired magnetic data. To design the new filter, we modified the space‐domain standard Wiener–Hopf filter by imposing two different constraints: (i) the filter energy constraint; and (ii) normalization of the filter coefficients. These constraints make it capable to characterize source bodies from their produced magnetic anomalies. We assume that the magnetic data are produced by induced magnetization only and interpretation can be as good as the subsurface model. Our technique is less sensitive to the data noise, which makes it efficient in enhancing the interpretation model. The modified filter demonstrates its applicability over the synthetic data with additive white Gaussian noise. In order to check the efficacy and adaptivity of this tool in a more realistic perspective, it is also tested on the real magnetic data acquired over a kimberlitic district adjoining to the western margin of the Cuddapah Basin in India to identify the source bodies from the anomalies. Our result shows that the modified Wiener–Hopf filter with the constraint for the magnetic data is more stable and efficient than the standard Wiener–Hopf filter.

Scaling Evidences of Thermal Properties in Earth’s Crust and its Implications

Fractal behaviour of the Earth’s physical properties has been discussed briefly in chapter 1. In this chapter, thermal properties of the Earth’s crust are analyzed and the significance of the results obtained is discussed. Here we redefine the traditional heat conduction equation for computation of geotherms by incorporating fractal distribution of thermal conductivity. Further, our study suggests the fractal distribution of radiogenic heat production rate inside the Earth, against the popularly used exponential and step models, which needs to be incorporated in the heat conduction equation.

Complexity in elucidating crustal thermal regime in geodynamically affected areas: A case study from the Deccan large igneous province (western India)

Complexity in the earth’s crustal structure plays an important role in governing earth’s thermal and geodynamic behavior. In the present study, an attempt has been made taking insights from our recent geological, geochemical, petrophysical and geophysical findings from specially drilled deep boreholes, to understand the lithospheric thermal evolution of the highly complex western India, which forms the core region of the Deccan large igneous province. This region was severely affected by the Deccan volcanic eruptions 65 Ma ago, which resulted in a totally degenerated, reworked and exhumed mafic crust, which presently contains several Tertiary basins with proven hydrocarbon reserves. Our detailed case study from the disastrous 1993 Killari earthquake (Mw 6.3) region, apart from some other geotectonically important localities like seismically active 2001 Bhuj and 1967 Koyna earthquake regions together with Tertiary Cambay graben, indicate that the western part of India, is perhaps one of the warmest segments of the earth. It is characterized by an average high mantle heat flow and Moho temperatures of about 43 mW/m2 (range: 31-65 mW/m2) and 660°C (range: 540-860°C) respectively. Estimated thickness of the lithosphere beneath these areas varies from as low as about 45 km to 100 km. Consequently, melting conditions in certain segments are expected at extremely shallow depths due to asthenospheric swell, like northern part of Cambay basin and Bhuj seismic zone beneath which only about half of original crystalline crust now remains due to sub-crustal melting and massive exhumation of deeper crustal layers. Sustained thermal heating and rise of isotherms appear to have resulted in substantial enhancement of hydrocarbon generation and maturation processes in Tertiary sediments. The present study highlights the need of an integrated geological, geochemical and geophysical study, if reasonably accurate deep crustal thermal regime is to be investigated.