Abstract
Several shallow gas kicks in Miocene have been encountered during drilling in North East Abu Dhabi (Ghantoot area). Gas origin is confirmed to be predominantly biogenic. ADNOC is evaluating subsurface potential as part of its strategy in developing prospective shallow gas accumulation. Tight layers are targeted to unlock potentially high amount of hydrocarbons and to achieve economical production targets. This paper demonstrates effectiveness of a modern reservoir-oriented technique for well and reservoir performance monitoring before and after stimulation jobs. This technique was proved to be effective at exploration stage when cost- and production-effective stimulation methods are analyzed and decided upon.
The spectral acoustic logging technique was applied to estimate inflow intervals in the tight gas reservoir, including pre- and post-stimulation monitoring. Spectral acoustic sensors record signals in a wide frequency range from 8 Hz to 60 kHz. Their dynamic range of 90 dB and large scanning radius allow accurate recording of relatively low-amplitude reservoir acoustic signals. Comprehensive analysis of the spectral acoustic data in combination with other logging techniques, such as temperature logging and a heat exchange sensor (a type of heat flow-meter) can be potentially useful for verification of complex, low-permeability reservoir parameters.
Shallow tight Gachsaran and Asmari biogenic gas formations are currently under appraisal targeting identification of highly potential zones and screening of production enhancement technics that allow achieving economical gas rates. Different stimulation technics were evaluated while testing of several exploration wells. One of the way to evaluate stimulation efficiency is an integrated logging that includes high-precision temperature logging and broadband high-sensitivity acoustic logging.
Several logging campaigns were conducted in exploration wells to evaluate well performance before and after different types of stimulation jobs: routine HCl stimulation, advanced chemical stimulation, mini- and propped hydraulic fracturing.
Due to the reservoir tightness, matrix flow is extremely weak and doesn’t allow sustaining the flow with or without nitrogen lifting that exclude the possibility of routine production logging with spinners. Using of High Precision Temperature (HPT) and Spectral Noise Logging (SNL) allows production profile evaluation for tight reservoir when survey is conducted after series of nitrogen lifting.
Due to the complexity of reservoir mineralogy (presence of clays, gypsum, anhydrites) HCl routine matrix treatment is found to be inefficient. Due to the reservoir tightness and based on logging and testing results, it was concluded that any types of matrix stimulation would not be efficient production enhancement technic for biogenic gas tight formations. Propped hydraulic fracturing allowed to bring gas to surface in the vertical well; sustainability of the flow needs to be evaluated in the horizontal well with propped stage fracking.
Differentiation between matrix and fracture flow was possible while interpreting noise amplitude and frequency; conducting HPT-SNL logging after propped hydraulic fracturing allows identification the direction of fracture propagation and level of containment within the target interval. HPT-SNL logging was proved to be effective at exploration stage when cost- and production-effective stimulation methods are analyzed and decided upon.
In tight gas reservoirs with high heterogeneity and mineralogy variation, it is challengeable to select proper enhancement technic allows achieving economical production rates. Selected logging techniques allowed identification of low rate flow intervals in tight gas reservoir and evaluation the efficiency of different stimulation techniques.
