Laminated sand-shale reservoir poses many challenges in formation evaluation and hence risks on decision making such as oil testing or completions, particularly for offshore. This paper presents a new approach of NMR based rock typing, permeability characterization and rapid production prediction for thin sand-shale laminated reservoir, conducted in an offshore field of South China Sea. The method is validated through a study integrating logs, formation tester and drill string test (DST) data, to build a generally applicable approach. It was used for rapid producibility evaluation right after openhole logging, in order to facilitate the decision making of the consequent completion and production test. Based on a quick yet reliable answer, operators can decide whether the well should be completed and where should be tested in order to achieve the lowest cost and HSE impact and better time efficiency for exploration and appraisal.The major challenges facing are: highly laminated sand/shale sequences, strong heterogeneities, complex porosity-permeability relationships, and different scales of different measurements. We set up an improved method using NMR measurements to evaluate effective properties within a depth frame of thin beds in order to overcome the resolution challenge because the individual lamination thickness is about centimeters or even thinner, which is much below the resolution of logs. Comparing with DST results, the accuracy of predicted producibility is in an acceptable range, giving great confidence of the reliability in the study result. This case study has demonstrated the applicability of the new NMR based method, and set up a rapid producibility evaluation approach for thin, sand-shale laminated reservoir; and this new NMR method and the approach applied are not limited to this specific field but applicable to other fields with similar geological settings and formation types in this region.

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Puguang is the biggest sour gas field in China with 17% H2S and 9% CO2 concentration and is the major gas sources for the pipeline from Sichuan to Shanghai. It's important to keep the gas wells healthily and steadily producing. However, the production is continually declining due to water coning, scale buildup and formation pressure dropping. It's urgent to perform well intervention effectively and economically to increase the well production. Production logging in multiple wells identified that inflow zones were less than 30% of perforation interval because of formation invasion and perforation tunnel plugging from initial completion. To release the potential productivity from nonproducing zones, reopening the perforations is critical to reverse the trend of production decline. To pass through the 3.5-in tubing with minimal restriction of 2.7-in and to effectively perforate the 7-in production casing and the carbonate formation, a 2.5-in spiral-exposed perforation system was finally selected as the optimized solution that can achieve the deepest formation penetration compared with conventional carrier system from simulations with rock-based perforation model. The spiral-exposed perforation system was modified to be H2S resistant, and wellbore dynamic simulation was conducted to minimize the gun shock to avoid gun drop during detonation. The detailed job design and risk mitigation plan were made as well to avoid any failure in such a highly corrosive environment with well depth up to 6,000 m. The first reperforation job was conducted successfully, and the well production increased 130,000 m3/day or 22.8%, and afterward, production logging confirmed that zonal production had significantly improved from the newly perforated interval. Then another seven wells were reperforated with an interval of 30 m to 45 m and average 100,000 m3/day gas production increment or 20% per well was achieved. It's proven that reperforation with redesigned exposed perforation system is the effective and economical way to increase production in the Puguang gas field. This paper illustrates how and what we do for exposed system improvement, candidate well selection, planning, and executing the through-tubing exposed perforation in such extremely challenging conditions. It also discusses lessons learnt and recommendations for future well intervention to steadily increase the well production in the Puguang gas field.

Integrated with core data, geological data and logging while drilling (LWD) borehole resistivity image and density-neutron logs, this paper demonstrated a field case study to characterize the heterogeneity of LHX-1 coral reef reservoir in South China Sea; which includes geological facies, lithology, fractures, secondary pore system etc. This reservoir is featured with hyper heterogeneity resulted by overlapped reefs, which have created vertical and lateral variation. High resolution LWD resistivity image logs were selected to address specific challenges and unknown: 1) appraisal well with limited data in place; 2) uncertainty regarding controlling factors on fracture and secondary pore system; 3) structural uncertainties including reservoir buried depth and thickness; and 4) uncertain controlling factors affecting original productivity and future production decline. The use of the integrated dataset was successful in delivering a full characterization of the reservoir heterogeneity: Fracture distribution was recognized and fairly understood in planar view and analyzed from qualitative and quantitative perspectives, secondary porosity was estimated and analyzed, and the controlling factors on productivity were clarified. The integrated analysis processes and expertise gained in this case study could be referred in other field development projects, not limited in carbonate reservoirs, but also in other heterogeneous reservoir such as turbidite or tight sand.

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