A new model is proposed for interpreting two-dimensional diffusion-relaxation nuclear magnetic resonance (2D NMR) maps by including the effects of restriction on the diffusion of water in a connected porous medium. The model's theoretical underpinnings are reviewed and a correlation derived between the diffusion and relaxation of restricted water molecules, i.e., the expected 'water line' where the water signal should appear on 2D NMR maps. The main application discussed in this paper is to improve the quantitative separation of different fluids in the formation, leading to better prediction of water and hydrocarbon saturations. In many carbonate formations, the effects of restriction may be quite pronounced making the standard fluid characterization with diffusion-relaxation maps questionable. The restricted-diffusion model is applied to laboratory measurements on carbonate core plugs and to an example log from a Middle East carbonate well where the saturations obtained with the new model are compared against other measures of saturation available in the dataset.

In a low-permeability Middle East carbonate reservoir, geologists, petrophysicists, and reservoir and drilling engineers had multiple requirements to optimally place the well, characterize and model fractures and faults, and evaluate the petrophysical attributes and cutoffs of the formation. A suite of logging-while-drilling (LWD) sensors was combined into a single bottomhole assembly (BHA) with the objective of acquiring the information needed to update the static model. An integrated study was then performed using production logs, which led to an improved dynamic model within this sector of the reservoir.An additional objective was to evaluate the contribution from each sensor for future applications in this field, or in similar fields. Based on this assessment, a workflow was agreed upon for optimizing the petrophysical data acquisition program and conveyance methods. From a scientific point of view, the comparison of the various logs and images helped to better understand the response of the various sensor measurements for this particular field.

The petrophysical evaluation for Sabkha environment is very challenge due to high reservoir heterogeneity and complex mineralogy in addition to existing of multi-lamination that have various thickness ranging from 1 to 20 ft., also these kinds of reservoirs normally have very high formation salinity that impact as well the water saturation calculations. In 2018, a unique worldwide biogenic shallow gas reservoir was discovered in Onshore Abu Dhabi, UAE in Miocene Sabkha deposits while drilling a dedicated well for deeper reservoir, the average reservoir gross thickness is about 4500 ft. with hundreds of intercalations from Clay, Limestone, Dolomite, Marl, Anhydrite and Halite. This paper presents the detailed challenges for such reservoir regarding the petrophysical evaluation (including but not limited to porosity calculations, water saturation evaluation by different methodology, net to gross ratio, log and core data integration, definition of high potential spot for testing, etc..) Moreover, the way to manage and mitigate these challenges in addition to testing technology and results. So far Five dedicated exploration wells were drilled in this reservoir at which a complete set of logs were acquired in addition to collecting of core footage about 4560 ft. from two wells and analysis is ongoing, also Rigless testing was done for 3 wells. In general, based on collecting data the reservoir properties showed very tight reservoir (permeability < 0.01 md) with high mineralogy complexity as well as high formation salinity (+ 300 kppm). Good Total Organic Carbone is also measures across the reservoir that confirmed the ability to produce Microbial gas. As a results of the integrated study, the sweet spots across the reservoir were identified and tested, accordingly placement for horizontal development wells as well as stimulation technology are optimized

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.

Recently there has been a growing interest in gas exploration, much of this focus has been directed toward thermogenic gas derived from cracking kerogen in the highly mature kitchens. However, a significant proportion of the global gas reserve is not thermogenic but of bacterial origin (Katz, 1995). Biogenic gas is an important exploration target because it occurs in geologically predictable circumstances, in areally widespread area, and in large quantities at shallow depths as free gas (Schneider et al, 2016). The recent exploration wells drilled in the northeast onshore Abu Dhabi showed elevated total gas readings during the drilling of the Gachsaran formation. Consequently, mud-weight was increased to control the gas flow. In addition, the recorded wireline logs indicate the presence of relatively high hydrocarbon saturations in several high porous zones of Gachsaran and Asmari formations. To assess the productivity and commerciality of the Biogenic gas potential in Abu Dhabi, several exploration wells are planned to be drilled before the end of 2019. The positive results of these wells will open the door for a new era of sweet gas exploration activities in Abu Dhabi and its surrounding areas. The primary gas reservoirs are thin carbonate and clastics layers in the Gachsaran Formation at a depth that ranges between 1600-5200 feet below sea level. Organic carbon isotopes, Rock Eval analysis, TOC log data and gas shows analysis indicated that the methane gas found in the Gachsaran Formation is of a biogenic origin and sourced mainly from the organic-rich argillaceous limestone of the Middle Gachsaran. Gachsaran formation is comprised of alternating thin layers of anhydrite, limestone, marl and shale sediments in addition to the presence of salt layers in the lower part. This mixed lithology resulted in the reservoirs property deterioration in particular by shale and anhydrite nodules cementation. The biogenic basin areal extent, significant thickness of the Gachsaran in this basin and the organic richness distribution, conclude possible generation of a huge volume of biogenic gas in northeast onshore Abu Dhabi. However, additional work is required to estimate the volume of gas that is accumulated and that can be produced from the Gachsaran and Asmari formations.