Petrophysical Characterisation of Gas Hydrates

Transcription

1 Petrophysical Characterisation of Gas Hydrates Anil Kumar Tyagi 1, Diya ukherjee 1 & Amuktha alyada 1 ABSTRACT The gas hydrates are crystalline substances composed of water and gas, in which each gas molecule is surrounded by a lattice structure of water molecules within the pore spaces of marine sediments. The gas molecule may be of methane, propane, ethane, butane or carbon dioxide gas. Natural Gas hydrates are known to occur worldwide, under certain temperature and pressure conditions in sediments of the outer continental margins, and in polar-regions associated with onshore and offshore permafrost. In deep-sea sediments, where temperature normally increases downward, a temperature eventually is reached at which the hydrate is unstable, even though the pressure continues to increase with depth. Thus a zone within the sediments exists in which gas hydrate is potentially stable from the seafloor down to a depth where the gas hydrate phase boundary is reached, commonly few hundred meters below the seafloor. Petrophysical properties of these sediments may vary depending upon the nature of the sediments. ost of the hydrates are associated with the claystone facies having very low resistivity. There are some places, where good sand has been reported, in those places resistivity goes very high. Petrophysical modeling in clay rich sediments is very important as the porosity computation, if not done properly without keeping in mind the unconsolidated nature of the sediments it will lead to overestimation of the resources. Current study demonstrates how the proper use of the technology can help not only in identifying but also in characterizing the hydrates. The study when constrained with core data, enhances the dependability of the results Keywords: Gashydrate, processing clay mineral, multineral INTRODUCTION In recent years, gas hydrates have drawn significant attention from all the scientific community worldwide due to their potential as an alternative energy source and a possible agent of shallow drilling hazards. Hydrates are solid compounds similar to ice-crystals, where the molecules of the natural gases are enclosed within the lattice structure of the water molecules (fig 1). 1 Reliance Industries Limited, Petroleum (E&P) The first evidence of such naturally occurring gas hydrate deposits was found in essiyokha field in Russian permafrost region. Subsequently gas hydrates were also found in shallow marine sediments of arctic region during various ODP legs in tropical deep-water areas where water depth exceeds meters. The present study is restricted only on marine gas hydrates of shallow offshore continental margin settings. The formation of gas hydrates depends on the presence of sufficiently high gas content, elevated pressure and high temperature (fig 2) which is of typical in the offshore continental margin settings, where formation of natural gases like methane is favored by rapid sedimentation and high content of sedimentary organic matter, providing extensive microbial fermentation. The most common occurrences of seafloor hydrate is in unconsolidated or semi-consolidated sediment pore spaces, although massive hydrate has been found as nodules and occasionally as thick veins or lenses. The study area is located in the central part of the eastern passive continental margin of India, where hydrates are present as disseminated form in the unconsolidated clay rich sediments at very shallow depth, few hundred meters below sea-floor. The bottom simulating reflector ( BSR) which is the result from the contrast between the high acoustic impedance of hydrate filled sediment overlying a lower impedance hydrate free zone demarcate the base of the hydrate stability zone. However, current studies has emphasized that the absence or presence of BSR may not be a direct indication of the hydrate presence in an area. Change in physical properties of the rocks due to the presence of hydrate and of underlying free gas can be easily detected by the well log data. The objective of the current study is quantification of physical properties of gas hydrates using the log and core data. 1

2 neutron porosities logs. The electrical resistivity and acoustic transit time also yield highly accurate gas hydrate saturations. Quality of the available downhole log data is one of the most significant factors controlling the accurate assessment of sediment porosities and gas hydrate saturations. Fig 1: Gas hydrates of type 1 structure, small spheres are tetrahedrally, linked water molecules which comprise the cages, large spheres are gas molecules. Resources associated in the present field of study come from the dispersed deposits of gas hydrate, generally found encased in fine-grained mud and shales. Gas hydrate occurrences are due primarily to extensive structural disturbance of the sediment. Such fractured reservoir accumulations may be common in certain areas, with thick sections exhibiting massive vein fills, or high concentrations of small hydrate nodules, smaller vein fills and massive layers parallel to bedding planes. Unlike the sand/sandstone systems where grain-supported reservoirs result in high matrix permeability and for which well-based production concepts are more plausible, extraction of methane from these shale-encased fractured accumulations will be very problematic. Gas hydrate responses of density and neutron tools are well established. The Neutron-Density tools cannot distinguish a gas hydrate from water, but are useful in determining porosity, clay-typing and detect free gas below reservoir. Porosities in gas hydrate zones are majorly affected by the type of hydrate accommodated itself in the pore spaces. ost of the studies from arctic and marine environments suggested that hydrates are present in the pore spaces but not as a coating for the grains. Hence, resistivity log responses in such areas are more or less comparable to that of oil or gas bearing sediment. Resistivity, here thus clearly demarcates gas hydrates from the host sediments. Fig 2: Stability and phase boundaries of gas hydrates superimposed on depth-temperature distribution in the ocean and upper sediment column. LOG BASED HYDRATE NTERPRETATION Gas volumes that may be attributed to a gas hydrate accumulation within a given geologic setting are dependent on a number of reservoir parameters two of which are sediment porosity and gas hydrate saturation which can be assessed with data obtained from well logging devices. The well logging devices that yield most accurate gas hydrate reservoir porosities include the gamma-gamma density log and NR measurements combined with the density is a good indicator of gas hydrate presence. They respond to hydrogen in pore fluids. Hydrates are solids, so they are invisible to NR tools. Thus, NR porosity represents non-gas-hydrate porosity. When compared to the density porosity log, the NR porosity can be used to compute accurate estimates of hydrate volume. This is called the density-magnetic resonance (DR) method (fig 3) The DR method also has limitations with two main potential sources of error. An incorrect matrix density will lead to errors in the density porosity computation. Also, when clay-bound water is present with fast relaxation times, the apparent NR porosity can be underestimated, particularly when the NR echo spacing is greater than 200 microseconds (µs). 2

3 main hole and the pilot hole over the interval which is shaded by yellow color shows that hydrates are not continuous body and are disseminated in nature. The absence of higher resistivity in the main hole in the above interval proves the above theory. Fig 3: DR method Saturations calculated here, in the present field of study, using resistivity provided the direct evidence that the occurrence of gas hydrate is partially controlled by mineralogy and lithology of the host sediments. From the available core data, it is observed that gas hydrates are present in the form of nodules, disseminated, which have been escaped through the fractures in the mud type clayey sediments The present study has utilized the multimineral approach for the estimation of petrophysical properties. The approach uses the following work flow: 1. Identification of hydrate zone using the log, drilling observations and core data. 2. Validation of hydrate zone using the pressure-temperature gradient, if it lies in hydrate-stability zone. 3. Zonation of the interval processed just as to remove the effect of formation compressional and mineralogical changes. 4. Parameter sensitivity analysis using the cross plot technique. 5. Identification of minerals based on core and cutting data. 6. Processing of data. Using the above approach data of the well A was processed. The well was drilled without riser and has difficulty while acquiring the log data. Before drilling the main hole, a pilot hole was drilled 50m away from the main well. Data was acquired in both the wells. Fig: 4 shows comparison of the pilot and main hole data. The difference in the resistivity between Fig 4: Comparison of resistivity and density data. Track 01: resistivity of main hole (RT_H) and resistivity of pilot hole (RT_PH) (shading shows difference in resistivity between main hole and pilot hole. Track 02: Depth in meters. Track 03: density of main hole (RHOB_H), density of pilot hole (RT_PH). Fig: 5 shows that the intervals shaded with multiple colors have higher resistivity in the range of ohmm which is more than the back ground resistivity of 1.15ohmm just above and below this zone. The decrease in density and neutron over the corresponding interval suggest the presence of gas hydrate in the zone. Fig 6 shows the core picture of the same zone. The white patches with in the core suggest the presence of hydrate in the zone. Image 3

4 RHOB (gm/cc) data (fig: 7) obtained over this interval also distinguishes the presence of gas hydrate. Log data was processed using the smectite as clay, due to its higher water affinity and low density (as in the case of our study). A simple quartz, smectite, water and gas model was run using the multimineral approach. As the reservoir is mostly shaly, it is difficult to estimate hydrate saturation using conventional Archies equation. Therefore, Dual water non-linear method is chosen for the estimation of saturation. Fig 08 shows the processed output of hydrate bearing zone. Gas saturation over the interval is around 20% and the total porosity estimated is around 30%. The higher water saturation is due to larger clay bound and capillary bound water present in the zone. Core data also suggest that this is a clay rich environment, therefore clay volume estimated is very high. Under these conditions the water saturation expected is also high. WELL-A RT (ohmm) Fig 5 Identification of hydrate zone with resistivity and density data and their cross plot. Track01: deep resistivity (red), shaded with orange colour with respect to back ground resistivity. Track02: depth in metres where hydrate zones are shaded with different colours. Track03: density (red).cross plot with deep resistivity (RT ohmm) on x-axis and density (RHOB in g/cc) on y-axis. Fig 6: Image of the core. Gas hydrates are present as disseminated form as shown in the figure by arrow. Fig 7: LWD image of the well A. Track 01: shows deep resistivity (red), shading with respect to shale base line (orange). Track 02: depth (meters). Track 03: density in red, track 04, 05: static and dynamic images of resistivity. Track 06, 07: static and dynamic images of density. Track 08: image of gamma ray. 4

5 Fig 8: ultimineral processed data of well-a..track01: caliper (black), shaded in yellow with respect to bit size. Track 02: GR (green). Track 03: depth (meters). Track 04: deep resistivity (red), shading with respect to shale base line (orange). Track 05: density (red), neutron porosity (blue). Track 06: total (maroon shading) and effective saturation (black). Track 07: total (green) and effective porosities (blue shading. rack 08: volumes of quartz, smectite, bound water, free water and gas. With similar ultimineral approach, well-b was processed and the output is shown in the fig: 9. The zone with resistivity in the range of ohmm, higher than the back ground resistivity of 1.15 ohm with corresponding lowering of density is identified as hydrate bearing. Gas saturation over this interval is ranging from 25-35% with a total porosity of 16-30%. Fig 9: ultimineral processed data of Well-B. Track 01: GR (green). Track 02: depth (meters). Track 03: deep resistivity (red), shading with respect to shale base line (orange). Track 04: density (red) shading with respect shale density (grey). Track 05: neutron porosity (blue). Track 06: total (green) and effective porosities (blue shading). Track 07: total (maroon shading) and effective saturation (black). Track 08: volumes of quartz, smectite, bound water, free water and gas. CONCLUSION Estimation of petrophysical properties of gas hydrate may not be a simple job. However with the integration of geological and drilling observations, it is possible to estimate the reservoir properties. Pressure testing carried out over the interval did not yield any movable hydrocarbon but with the reduction of mud pressure, gas 5

6 bubbles could be observed on seafloor, confirming the mobility of dissociated hydrated gas. Efforts are to be made to design the proper completion strategy for the production of hydrates. ACKNOWLEDGEENT We sincerely thank Reliance Industries Limited (E & P) for providing the technical guidance, support and assistance to publish this paper REFERENCES Doug urray, asa fumi Fukuhara, Osamu Osawa and TatsukiEndo, Rober Kleinberg and Bikash Sinha, Takatoshi Namikawa, Charecterising gas hydrated reservoirs, E&P, A Hart Energy Publication, J.Dai and N.Dutta, Exploration for Gas Hydrates Based on Seismic Information With Examples from Northern Gulf of exico, EAGE 69th Conference & Exhibition London, UK, June athew K. Davie and Bruce A. Buffett, A numerical model for the formation of gashydrate below the seafloor, Rock the Foundation Convention, Canadian Society of Petroleum Geologists, June 18-22, 2001, Abstract p Ray Boswell and Tim Collett, Fire in the ice, ethane hydrate News letter, Reprinted from the Fall 2006, Department of energy, U.S.A, NETL. Timothy S. Collett, Quantitative Well-log analysis of in-situ natural Gas hydrate, thesis. Timothy S. Collett, Scott R. Dallimore, Integrated Well log and Reflection Seismic Analysis of Gas hydrate accumulations, Rock the Foundation Convention, Canadian Society of Petroleum Geologists, June 18-22, Umberta Tinivella and Jose.Carcione Estimation of gas-hydrate concentration and free-gas saturation from log and seismic data, The Leading Edge Journal, February 2001, p ABOUT THE AUTHORS Anil Kumar Tyagi is working as Asstt. Vice president with Reliance Industries Limited. He has got an experience of 27 years as Petrophysicist. He has expertise in carbonate as well as clastic reservoir. He holds masters degree from Indian Institute of Technology, Roorkee, India. He has been responsible bringing in the concept of resistivity anisotropy to Indian context. The concept has helped in unlocking the potential of thin bedded reservoir form the deep water basins of India. He is member of SPWLA, SPE and SPG Diya ukherjee, a Petrophysicist by profession has been working with Reliance for the past one and half years. She completed her.sc. in geology from Indian Institute of technology Kharagpur. A clear basic knowledge about petrophysics in academics combined with their practical applications on ongoing projects made her to target the study of gas hydrates and excel their interpretation. Amuktha alyada, a petrophysicist by profession has been working with Reliance for the past one and half years. She completed her.sc. in geophysics and Bachelors in aths, physics and chemistry from Osmania University, AP. Her present work in Reliance includes real time monitoring and petrophysical interpretation of Krishna and Godavari and Cambay basin data. A long stay in this gas hydrate project along with timely guidance and repository of available resources helped her to prove her mettle, by bringing out some unconventional approaches and finish this project effectively. 6