Dominating factors on well productivity and development strategies optimization in Weiyuan shale gas play, Sichuan Basin, SW China

Weiyuan shale gas play is characterized by thin high-quality reservoir thickness,big horizontal stress difference,and big productivity differences between wells.Based on integrated evaluation of shale gas reservoir geology and well logging interpretation of more than 20 appraisal wells,a correlation was built between the single well test production rate and the high-quality reservoir length drilled in the horizontal wells,high-quality reservoir thickness and the stimulation treatment parameters in over 100 horizontal wells,the dominating factors on horizontal well productivity were found out,and optimized development strategies were proposed.The results show that the deployed reserves of high-quality reservoir are the dominating factors on horizontal well productivity.In other words,the shale gas well productivity is controlled by the thickness of the high-quality reservoir,the high-quality reservoir drilling length and the effectiveness of stimulation.Based on the above understanding,the development strategies in Weiyuan shale gas play are optimized as follows:(1)The target of horizontal wells is located in the middle and lower parts of Longyi 11(Wei202 area)and Longyi 11(Wei204 area).(2)Producing wells are drilled in priority in the surrounding areas of Weiyuan county with thick high-quality reservoir.(3)A medium to high intensity stimulation is adopted.After the implementation of these strategies,both the production rate and the estimated ultimate recovery(EUR)of individual shale gas wells have increased substantially.

Methods for Shale Gas Play Assessment: A Comparison between Silurian Longmaxi Shale and Mississippian Barnett Shale

Based on field work, organic geochemical analyses and experimental testing, a six-property assessment method for shale gas is proposed. These six properties include organic matter properies, lithofacies, petrophysical properties, gas content, brittleness and local stress field. Due to the features of continuous distribution over a large area and low resource abundance in shale plays, a sweet spot should have these following properties： （a） TOC〉2%; （b） brittle minerals content （〉40%） and clay minerals （〈30%）; （c） Ro （〉1.1%）; （d） porosity （〉2%） and permeability （〉0.000 1 mD）, and （e） effective thickness （30-50 m）. Applying these criteria in the Sichuan Basin, the Silurian Longmaxi shale consists of four prospecting sweet spots, including blocks of Changning, Weiyuan, Zhaotong and Fushnn-Yongchuan. Although these four blocks have some similarities, different features were usually observed. After comprehensive analyses using the six-property assessment method, the Fushun-Yongan Block ranks the most favorable sweet spot, followed by the Weiyuan Block. For the other two blocks, the Changning Block is better than the Zbaotong Block. By comparing with the Mississippian Barnett shale, characteristics that are crucial for a high-yielding in the Sichuan Basin include a high content of organic matter （TOC〉2.5%）, a moderate thermal maturity （Ro=0.4%-2%）, a high content of brittle minerals （quartz： 30%-45%）, a high gas content （〉2.5 m^3·t^-1）, and types I and II1 kerogen.

Enrichment conditions and resource potential of coal-rock gas in Ordos Basin,NW China

To reveal the enrichment conditions and resource potential of coal-rock gas in the Ordos Basin,this paper presents a systematic research on the sedimentary environment,distribution,physical properties,reservoir characteristics,gas-bearing characteristics and gas accumulation play of deep coals.The results show that thick coals are widely distributed in the Carboniferous–Permian of the Ordos Basin.The main coal seams Carboniferous 5~#and Permian 8~#in the Carboniferous–Permian have strong hydrocarbon generation capacity and high thermal evolution degree,which provide abundant materials for the formation of coal-rock gas.Deep coal reservoirs have good physical properties,especially porosity and permeability.Coal seams Carboniferous 5^(#)and Permian 8^(#)exhibit the average porosity of 4.1%and 6.4%,and the average permeability of 8.7×10^(-3)μm^(2)and 15.7×10^(-3)μm^(2),respectively.Cleats and fissures are developed in the coals,and together with the micropores,constitute the main storage space.With the increase of evolution degree,the micropore volume tends to increase.The development degree of cleats and fissures has a great impact on permeability.The coal reservoirs and their industrial compositions exhibit significantly heterogeneous distribution in the vertical direction.The bright coal seam,which is in the middle and upper section,less affected by ash filling compared with the lower section,and contains well-developed pores and fissures,is a high-quality reservoir interval.The deep coals present good gas-bearing characteristics in Ordos Basin,with the gas content of 7.5–20.0 m^(3)/t,and the proportion of free gas(greater than 10%,mostly 11.0%–55.1%)in coal-rock gas significantly higher than that in shallow coals.The enrichment degree of free gas in deep coals is controlled by the number of macropores and microfractures.The coal rock pressure testing shows that the coal-limestone and coal-mudstone combinations for gas accumulation have good sealing capacity,and the mudstone/limestone(roof)-coal-mudstone(floor)combination generally indicates high coal-rock gas values.The coal-rock gas resources in the Ordos Basin were preliminarily estimated by the volume method to be 22.38×10^(12)m^(3),and the main coal-rock gas prospects in the Ordos Basin were defined.In the central-east of the Ordos Basin,Wushenqi,Hengshan-Suide,Yan'an,Zichang,and Yichuan are coal-rock gas prospects for the coal seam#8 of the Benxi Formation,and Linxian West,Mizhi,Yichuan-Huangling,Yulin,and Wushenqi-Hengshan are coal-rock gas prospects for the coal seam#5 of the Shanxi Formation,which are expected to become new areas for increased gas reserves and production.