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Logging-While-Drilling (LWD) has incorporated almost all wireline-equivalent technology with the added advantage of logging high-angle and horizontal wells with reduced rig time, critical for cost optimization efforts. LWD measurements are affected by a rugged drilling environment, and logging interpretation with a wireline mindset leads to erroneous results. Identifying measurement artefacts from real formation information is critical for reliable log analysis. This publication discusses the most common effects of drilling dynamics and environments on LWD logs that were observed during logging and drilling wells in cretaceous carbonate reservoirs in an Abu Dhabi onshore field. Log data from more than one hundred wells are reviewed to identify several interesting effects due to bottom-hole-assembly (BHA) design, BHA driving mechanism (Rotary steerable system versus mud motor), tool eccentricity, well angle, mud properties, differential invasion, borehole condition, formation fluid properties and lithology. In a few instances, some of these effects occur simultaneously, complicating the log response. These phenomenons are discussed in detail with actual examples and compared to offset wells and response modellings. The rugged logging environment and limited formation damage due to invasion provide a unique opportunity to obtain additional insight about reservoir behavior, especially when compared to wireline data in an offset well or in the same well. Pre-job planning and modelling can use these phenomena for getting additional information about dynamic reservoir behavior. This paper highlights a few such applications. This paper explains the impact of a dynamic drilling environment on LWD measurements and serves as a ready reference to identify measurement artifacts from real formation information. It is helpful as a guidebook for log analysts, geologist, geo-steering engineers and other non-specialists to identify LWD measurement artefacts.

Reservoir A is an Upper Jurassic reservoir in offshore Abu Dhabi, composing layers of dense anhydrite and porous mixed lithology of dolomite and limestone. Petrophysical study from multiple wells suggests that the rock quality within the reservoir has significant lateral and vertical variations that can result in different flow capacities. Consequently, it is crucial to identify the rock quality variations and the consequent flow capacity in horizontal wells to optimize development plan, ideally in real-time. However, these lateral and vertical variations are not visible from conventional porosity (density / neutron) logs, making identification of rock quality very challenging. This paper introduces an innovative magnetic resonance (NMR)-based real-time method of permeability prediction and rock typing. Wireline logs including NMR were acquired in a pilot well, providing porosity and extensive T2-based information (permeability index, irreducible and movable fluid volume and porosity partition). Routine core analysis was also available to calibrate the NMR data, achieving a suitable correlation for NMR permeability index calibration in this field. Several rock types could be identified with the Windland R35 technique using porosity and calibrated permeability from NMR. This identification was then validated by rock types from cores. The application of knowledge gained from the study led to advanced reservoir characterization solely based on the NMR log. The process was applied to high-angle and horizontal (HAHZ) wells where the NMR full-spectrum log while drilling was available. Several slanted wells were drilled with a fit-for-purpose logging-while-drilling (LWD) suite including NMR for geo-steering and formation evaluation. The real-time LWD NMR data helped trace a remarkable change of irreducible water level through certain layers, suggesting that the subzones of Reservoir A changed pore geometry and rock type laterally, resulting in variations of flow capacity and reservoir performance. In one example, this method indicated unexpected good rock quality in one of these subzones considering the experience from offset well. Subsequently, the LWD formation-testing tool confirmed the result with mobility measurements, proving the NMR-based methodology was valid. This process normally applies to memory data after drilling, playing a key role in designing completion strategy in a timely manner. The process is also available in real-time while drilling if full NMR data is transmitted to surface, serving as a safer logging-tool for identification of sub-zones with additional valuable information compared to regular porosity tools with chemical radioactive source.

Shale gas reservoirs are characterized in low gas abundance, poor permeability, lower natural productivity than the lower limit of industrial oil flow, and rapid formation energy decline. At present, the technology of horizontal well drilling and staged hydraulic fracturing is widely used for the exploitation of such low-porosity and low-permeability reservoirs. The long well section of the horizontal well in the reservoir and the hydraulic fractures formed by fracturing act as the "underground expressway" for the deep gas in the reservoir to flow toward the wellbore. Their combination can greatly increase the production performance of the oil and gas resources in the reservoir. Staged multi-cluster fracturing in horizontal wells is the key technology to achieve the profitable shale gas production. The results of on-site downhole perforation imaging and distributed optical fiber temperature and acoustic monitoring show that there are obvious non-uniform liquid inflow and expansion phenomena in each cluster of fractures during the fracturing process. Relevant research results also show that factors such as the heterogeneity of the reservoir and the stress interference caused by the propagation of multiple fractures are the main causes of the non-uniform propagation of hydraulic fractures. Therefore, it is accessible to simulate the complex balanced expansion of each cluster of fractures in the fracturing section to improve the coverage of hydraulic fractures in the horizontal well section with numerical simulation methods based on the basic theory of elasticity and fracture mechanics, to reveal how the above engineering geological factors influence and control the fracture propagation. The results of the simulation of the fracturing treatment section of the deep shale gas horizontal well by the fracture propagation model are consistent with the micro-seismic monitoring results,which has obvious significance for accelerating the exploitation of difficult-to-exploit resources and guaranteeing the supply of gas resources.

Since the tight sandstone gas reservoir which is widely distributed in China has become increasingly important in oil field with the further development of resource, it is extraordinarily meaningful for the sustainable and healthy development of China’s energy industry to explore its benefit development mode. Though great achievements have made with the wide use of the hydraulic fracturing technology, which serves as an effective process measure to increase the productivity of a single well, in the development of global unconventional oil and gas resources, there have been many technical problems exposed. A critical one is that if the fractured stage length is too long, the oil and gas resources won’t be effectively exploited, and if it is too short, the operation cost and time will be increased apparently. Therefore, it is urgently required to make plans for determining the optimal length of the fractured reservoir based on different geological features of the oil and gas reservoirs. This paper took the tight oil reservoir in Lower Wuerhe formation in study area as the research case, determined 5 fracturing stage length cases combined with the treatment status and pumping injection procedure of M oil field: Case A (40m), Case B(50m), Case C (60m), Case D (70m) and Case E(80m), and realized fully 3D coupled simulation of the hydraulic fractures in H1 well based on the 3D geomechanical modeling and 3D DFN model with considering multiple factors including stress shadow, proppant settlement and migration using the unstructured grid technology to preprocess it to improve the capacity prediction accuracy of numerical simulation. The productivity prediction results showed that the 10-year EUR (Estimated Ultimate Recovery) of a single well ranged from 35,500 tons to 48,200 tons. With the comprehensive production and fracturing operation cost being considered comprehensively, it was recommended that the optimal length of the single fractured reservoir should be 60 meters.