Developmental stage related patterns of codon usage and genomic GC content: searching for evolutionary fingerprints with models of stem cell differentiation

Developmental-stage-related patterns of gene expression correlate with codon usage and genomic GC content in stem cell hierarchies

Impact of fracturing liquid absorption on the production and water-block unlocking for shale gas reservoir

A large amount of liquid is pumped into the shale gas reservoir during hydraulic fracturing, and the fluid flowback ratio is usually low. However, field experience showed that the liquids did not cause severe damage to shale gas reservoir. It is urgent to clarify the water block unlocking mechanism of a shale gas reservoir. This work is to discuss the water block unlocking mechanism in shale gas reservoir. Based on the characteristic study of shale gas formation, the fracturing fluid absorption mechanism, absorption ability and impact on shale gas formation damage are systematically studied. Study shows that ultra-low water saturation, abundant micro- to nano- tubulars and a huge contact area are the control factors for strong fluid absorption ability of gas-shale. The strong water absorption capacity of the shale gas formation matrix is a key factor in removing water block. Organic matter also has an important influence on absorption ability and gas production. A conceptual evaluation criterion for water block unlocking is proposed based on core absorption capacity, original water saturation and fracture density. The shut-in after hydraulic fracturing is beneficial to gas production and can reduce water production for certain shale gas reservoir.Cited as: Shen, Y., Ge, H., Zhang, X., Chang, L., Liu, D., Liu, J. Impact of fracturing liquid absorption on the production and water-block unlocking for shale gas reservoir. Advances in Geo-Energy Research, 2018, 2(2): 163-172, doi: 10.26804/ager.2018.02.0

Experimental Investigation on Organic Matter Orientation Characteristics of Terrestrial and Marine Shale in China

As an essential component in shale, OM (organic matter) grains and their arrangements may play essential roles in affecting the anisotropy of the reservoir. However, OM grains are commonly treated as an evenly distributed isotropic medium in current studies, and few works have been done to investigate their detailed arrangement characteristics. In this study, terrestrial and marine shale samples were collected from three different shale plays in China, and the arrangement characteristics of OM grains in each sample were investigated by SEM (scanning electron microscope) image analysis. The results indicate that OM grains in shale are not evenly distributed in isotropic medium, and their directional alignment is pervasive in both marine and terrestrial shale. OM grains in shale tend to subparallel to the bedding section, and their orientation degree and controlling factors differ among different shales. OM grains in samples from terrestrial C-7(Chang-7 Formation) exhibit the strongest directionality in their arrangement, and OM grains in samples from marine LMX (Longmaxi Formation) shale in the Fuling area also exhibit strong directional alignment. While in samples from marine LMX shale in the Baojing area, their directional alignment is much weaker. Shales with high clay content, high TOC (total organic carbon), low thermal maturity, and flat reservoir structure get more OM grains parallel to the bedding section. The biogenetic texture of graptolite in marine LMX shale is the dominating factor leading to the strong directional alignment of the OM grains. However, syncline structure may disorganize the preformed directional alignment and weaken the directionality of the OM grains, which results in the OM arrangement difference between LMX samples from Fuling and Baojing. While the compaction of the layered clay particles is the dominating mechanism leading to the strong directional alignment of the OM grains in terrestrial shale samples from C-7

A new tool-less layered fracturing technology and its pilot application in deep thick formations

Multi-layer fracturing technology is challenging because of the risk of packer failure and high cost in the deep thick formation. It depends largely on the effectiveness of packer tools. However, a new degradable fiber ball could be successfully used to temporarily block the open perforations, and then the layer with higher fracturing pressure is broken down. This paper presents a new tool-less layered fracturing technique and its pilot test results with this special material. A series of laboratory experiments were conducted to evaluate the feasibility of this new technique. Degradable fiber balls were applied to perforated pipes under simulated reservoir conditions. The ball carried by the fluid first sealed the perforation holes and then increased the pressure in the pipe to simulate the resistance to pressure. In addition, the fluid was heated up to 140¿ to simulate the degradation rate of fiber balls. Throughout these processes, the flow rate, temperature and pressure were continuously monitored for subsequent analysis. Experimental and application results showed that: (1) fiber balls could be thoroughly degraded at 140¿ temperature after six hours; (2) at a pressure difference of 50-70MPa, its deformation rate was less than 1.5%, which indicated its higher compression capability; (3) it could effectively block the perforation holes at 90¿ and a pressure difference of 20MPa; (4) The blockage of perforations by the fiber ball could significantly enlarge the net pressure in the wellbore. This technique was applied for 35 wells in a deep and thick oil reservoir, which had achieved a great success and the post-treatment oil production was enhanced by 50-60% compared with conventional stimulation techniques.Postprint (published version

The effect of stress on limestone permeability and effective stress behavior of damaged samples - original data used for figures

The evolution of permeability and its effective stress behavior is related to inelastic deformation and failure mode. This was systematically investigated in Indiana and Purbeck limestones with porosities of 16% and 14%, respectively. High‐pressure compression tests were conducted at room temperature on water‐saturated samples. At relatively high confinement shear‐enhanced compaction was observed to initiate at a critical stress, accompanied by significant permeability reduction of up to a factor of ~3. Overall, the permeability reduction due to inelastic compaction in our limestones is smaller than that observed in sandstones. At relatively low confinement, dilatant failure was observed, which was accompanied by a decrease and increase of permeability in Indiana and Purbeck limestones, respectively. There seems to be a trend for the correlation between porosity and permeability changes to switch from positive to negative with increasing porosity. The void space of both limestones has significant proportions of macropores and micropores. The effective stress behavior of such a limestone with dual porosity has been documented to be different from the prediction for a microscopically homogeneous assemblage, in that its effective stress coefficients for permeability and pore volume change may attain values significantly >1. In contrast, our investigation of damaged samples consistently showed effective stress coefficients for both permeability and pore volume change with values <1. This suggests that the behavior in the damaged samples is akin to that of a microscopically homogeneous assemblage, possibly due to pervasive collapse of macropores that would effectively homogenize the initially bimodal pore size distribution

A new tool-less layered fracturing technology and its pilot application in deep thick formations

Multi-layer fracturing technology is challenging because of the risk of packer failure and high cost in the deep thick formation. It depends largely on the effectiveness of packer tools. However, a new degradable fiber ball could be successfully used to temporarily block the open perforations, and then the layer with higher fracturing pressure is broken down. This paper presents a new tool-less layered fracturing technique and its pilot test results with this special material. A series of laboratory experiments were conducted to evaluate the feasibility of this new technique. Degradable fiber balls were applied to perforated pipes under simulated reservoir conditions. The ball carried by the fluid first sealed the perforation holes and then increased the pressure in the pipe to simulate the resistance to pressure. In addition, the fluid was heated up to 140¿ to simulate the degradation rate of fiber balls. Throughout these processes, the flow rate, temperature and pressure were continuously monitored for subsequent analysis. Experimental and application results showed that: (1) fiber balls could be thoroughly degraded at 140¿ temperature after six hours; (2) at a pressure difference of 50-70MPa, its deformation rate was less than 1.5%, which indicated its higher compression capability; (3) it could effectively block the perforation holes at 90¿ and a pressure difference of 20MPa; (4) The blockage of perforations by the fiber ball could significantly enlarge the net pressure in the wellbore. This technique was applied for 35 wells in a deep and thick oil reservoir, which had achieved a great success and the post-treatment oil production was enhanced by 50-60% compared with conventional stimulation techniques

Experimental Studies on Shale Cracks and Permeability Evolution Based on Acoustic Emission Monitoring

The matrix permeability of shale reservoirs is extremely low. Therefore, massive volume fracturing is needed to form a complex crack network and get adequate sufficient capacity during the well completion. After fracturing, the effective stimulated reservoir volume (ESRV) is vital for developing shale reservoirs, mainly determined by stimulated reservoir volume (SRV) and the increase in permeability. Microseismic monitoring is widely used in the field to describe the crack shape and determine the SRV, to evaluate the stimulation effect. However, no studies have been conducted on the relationship between microseismic parameters and permeability. Thereby, we conducted uniaxial compression tests on Longmaxi shale samples and measured their changes in porosity and permeability before and after loading combining the microseismic monitoring under a laboratory scale (acoustic emission (AE)). Results show that porosity has little influence on the permeability before and after loading, while the propagation and connection of cracks are the most critical factors. As the loading stress increases, the crack volume and sample connectivity both grow. Besides, for the Longmaxi shale, when the stress is loaded to 30~50% of uniaxial compressive strength (UCS), the cracks start to propagate steadily (dilation), the permeability begins to increase rapidly, and percolation occurs, which indicates that the dilation point is closely related to the percolation threshold. The AE rate and accumulative ringing number both increase when it is larger than the percolation threshold value. The variation of AE characteristics can be used to identify the percolation threshold. Finally, the graphic model including AE parameters, crack, and permeability evolution is established based on the experimental results, which could help us understand the relationship between microseismic parameters and permeability and provide a methodological basis for the ESRV evaluation in the field

Ion Diffusion Behavior between Fracturing Water and Shale and Its Potential Influence on Production

Water imbibition, conductivity measurements, and ion identification were performed to investigate ion diffusion behavior between slick water and shale for large-scale hydraulic fracturing. The results indicated that there was strong ion exchange between water and shale. The ion concentration in water increases with fracture complexity and is dependent on the salinity of fracturing fluids. This implies that fracturing effects could be forecast from flow-back fluid ion concentrations after large-scale slick water fracturing. Higher levels of ion diffusion imply the presence of larger fracturing areas and higher level of fracture density for a similar reservoir. The mechanism of ion diffusion and the corresponding effects on IOR (increased oil recovery) based on a field example are discussed

Experimental study on the mobility of Junggar Basin's Jimsar shale oil by CO2 huff and puff under different temperatures and pressures

Micro- and nano-scale pore-throat fissure systems were mainly developed in the Jimsar shale oil reservoir of the Junggar Basin with the oil of viscous and difficult to be produced.CO2 huff-and-puff is an important technology to enhance the oil recovery. To understand the mobility law of Jimsar shale oil reservoir under CO2 huff and puff, 45 cores of the Lucaogou Formation in this area were studied in this study.The cores was classified into dolomitic sandstone, doloarenite and lithic sandstone. The overburden porosity of the reservoir is 2.0%-22.7%, and the average value is only 11.0%. The average overburden permeability is 0.01×10-3 μm2, and more than 90% of the samples have permeability lower than 0.1×10-3 μm2. According to physical property classification, 20 rock samples were further selected and 6 key parameters for low-field NMR measurement were optimized. By comparing the experimental data of shale oil mercury injection with those of low-field NMR, the linear relationship between T2 value and pore radius of shale core was established in logarithmic coordinates.The pore radius distribution of shale was obtained quantitatively according to the T2 spectrum. 9 kinds of CO2 huff and puff experiments were carried out under different temperatures and pressures. The analyses of recovery rate, utilization degree and other indicators show that shale oil in small pores(r 1 000 nm) is relatively higher, and increases with the increase of temperature and pressure