Author : Wendy Young
Publisher :
ISBN 13 :
Total Pages : 164 pages
Book Rating : 4.:/5 (93 download)
Book Synopsis Feasibility of Time-lapse Gravity Monitoring of Gas Production and CO2 Sequestrartion, Northern Carnarvon Basin, Australia by : Wendy Young
Download or read book Feasibility of Time-lapse Gravity Monitoring of Gas Production and CO2 Sequestrartion, Northern Carnarvon Basin, Australia written by Wendy Young and published by . This book was released on 2012 with total page 164 pages. Available in PDF, EPUB and Kindle. Book excerpt: [Truncated abstract] Time-lapse (4D) seismic data, often used to monitor hydrocarbon production and CO2 injection in subsurface reservoirs, cannot readily detect gas saturation changes under certain conditions. Seismic data respond primarily to variations in the compressibility of a rock, but for gas-fluid mixtures greater than ~20%, a change in gas saturation may cause only a minimal change in the compressibility of the reservoir rock. Therefore, it can be difficult to discriminate reservoirs with high and medium gas saturations using the seismic technique. To better monitor changes in reservoir gas saturation, a non-seismic technique may be more favourable. Complementary geophysical techniques, such as gravity and electromagnetic (EM) methods, respond to subsurface variations in density and resistivity respectively, and these physical properties are highly dependent on the saturation values of the rock's pore fluids. Compared to 4D seismic surveys, time-lapse gravity and EM acquisition costs have the potential to be less expensive; however, they also contain less spatial resolution. Gravity data has an additional benefit of being linearly proportional to changes in mass/density, and thus may be easier to interpret than alternate geophysical data types. To detect small mass changes in offshore subsurface reservoirs requires high precision gravity data, which can be achieved by accurate repositioning of the gravimeters on the seafloor. After applying a variety of data corrections, the change in the gravity signal over time can be related to variations in the fluid saturations or pore pressures in the subsurface reservoir. The time-lapse gravity signal may be particularly useful because the amounts of aquifer influx and/or pressure depletion in an offshore reservoir are key uncertainties impacting ultimate gas recovery. The objective of my thesis research is to develop and perform a feasibility analysis for time-lapse gravity monitoring of gas production and CO2 injection in Northern Carnarvon Basin reservoirs. To do this, I have developed a method to quickly assess the sensitivity of time-lapse gravity measurements to reservoir production or injection related changes. I show that gravity monitoring of Carnarvon gas reservoirs appears to be technically feasible and encourages further detailed assessment on a field-by-field basis. For example, in a strong water-drive scenario, a field-wide height change in the gas-water contact greater than 5 m can produce a detectable gravity response greater than 10 U+006fGal for a reservoir at a depth of 2 km, with a porosity of 0.25 and a net-to-gross sand ratio of 0.70. Alternately, for the same reservoir in a depletion-drive scenario, a 6 MPa (~900 psi) decrease in pressure throughout the field can also produce a detectable gravity response. To monitor CO2 sequestration using the time-lapse gravity technique, I find that it is easier to detect a CO2 plume (or potential leaks) in shallower formations (at or less than 1 km below mudline) compared to deeper storage formations at depths greater than 2 km. In order to produce a detectable gravity anomaly, significant amounts of CO2 in excess of 4-8 MT must be injected for reservoirs at 2 km depth, compared to only 1 MT of CO2 injection for formations at 1 km depth. The methods I have developed to assess the feasibility of gravity monitoring are both flexible and practical...
