Ultra-low Permeability Sandstone Reservoir Research Articles

Simultaneous hydraulic fracturing of ultra-low permeability sandstone reservoirs in China: Mechanism and its field test

Based on the impact of the stress perturbation effect created by simultaneous propagation of multiple fractures in the process of simultaneous hydraulic fracturing, a thorough research on the mechanism and adaptation of simultaneous fracturing of double horizontal wells in ultra-low permeability sandstone reservoirs was conducted by taking two adjacent horizontal wells (well Yangping-1 and well Yangping-2 located in Longdong area of China Changqing Oilfield) as field test wells. And simultaneous fracturing optimal design of two adjacent horizontal wells was finished and employed in field test. Micro-seismic monitoring analysis of fracture propagation during the stimulation treatment shows that hydraulic fractures present a pattern of complicated network expansion, and the well test data after fracturing show that the daily production of well Yangping-1 and well Yangping-2 reach 105.8 t/d and 87.6 t/d, which are approximately 9.4 times and 7.8 times the daily production of a fractured vertical well in the same area, respectively. Field test reflects that simultaneous hydraulic fracturing of two adjacent horizontal wells can enlarge the expansion area of hydraulic fractures to obtain a lager drainage area and realize the full stimulation of ultra-low permeability sandstone reservoirs in China Changqing oilfield. Therefore, simultaneous fracturing of two adjacent horizontal wells provides a good opportunity in stimulation techniques for the efficient development of ultra-low permeability reservoirs in China Changqing oilfield, and it has great popularization value and can provide a new avenue for the application of stimulation techniques in ultra-low permeability reservoirs in China.

The tectonic fracture modeling of an ultra-low permeability sandstone reservoir based on an outcrop analogy: A case study in the Wangyao Oilfield of Ordos Basin, China

Due to inherent limits of data acquisition and geophysical data resolution, there are large uncertainties in the characterization of subsurface fractures. However, outcrop analogies can provide qualitative and quantitative information on a large number of fractures, based on which the accuracy of subsurface fracture characterization can be improved. Here we take the tectonic fracture modeling of an ultra-low permeability sandstone reservoir based on an outcrop analogy, a case study of the Chang611 Formation of the Upper Triassic Yanchang Group of the Wangyao Oilfield in the Ordos Basin of China. An outcrop at the edge of the basin is a suitable analog for the reservoir, but the prerequisite is that they must have equivalent previous stress fields, similar final structural characteristics, relative timing and an identical depositional environment and diagenesis. The relationship among fracture density, rock type and bed thickness based on the outcrop is one of the most important fracture distribution models, and can be used to interpret fracture density in individual wells quantitatively. Fracture orientation, dip, geometry and scale, also should be described and measured in the outcrop, and can be used together with structure restoration and single well fracture density interpretation to guide fracture intensity prediction on bed surfaces and to constrain the construction of the 3D fracture geometry model of the subsurface reservoir. The application of the above principles shows the outcrop-based tectonic fracture models of the target ultra-low permeability sandstone reservoir are consistent with fractures inferred from microseismic interpretation and tracer tests. This illustrated that the fracture modeling based on the outcrop analogy is reliable and can reduce the uncertainty in stochastic fracture modeling.

The Relationship of Ultra-Low Permeability Sandstone Aspect Ratio With Porosity, Permeability

The ultra-low permeability sandstone reservoir has large aspect ratio which significantly influences the multi-phase percolation characteristic. The ratio could be accurately measured by rate-controlled mercury porosimetry, but the testing technology is expensive, time-consuming and core-contaminating. There is not a simple effective method to describe the aspect ratio. The pores of the ultra-low permeability sandstone are mainly connected by the very long narrow throats, which could be advantageously simulated by the compound capillary bundles model. The analytical expressions of porosity and permeability about major pore structure parameters are established based on the model for the tight porous media. After solving the two expressions, the relationship between aspect ratio and parameter combination of porosity, permeability is obtained for the ultra-low permeable sandstone. Then the relation is fitted in this article using many previous published rate-controlled mercury data on compact sandstone and the relevance is strong, which proves that aspect ratio of tight rock is able to be calculated with its porosity and permeability. Key words : Ultra-low permeability sandstone; Aspect ratio; Pore; Throat; Porosity; Permeability

Analysis on Characteristics of Clay Minerals of Ultra-Low Permeability Sandstone Reservoir and Potential Damage to Reservoir

By X-ray diffraction analysis, scanning electron microscopy and other research methods, this paper detailedly analysizes types and micro-characteristics of clay minerals in the Ordos Basin, Chang 8 formation of Xifeng Oilfield. Studies suggest that clay minerals mainly include petal-like and pompon chlorite, bridge-like and curved sheet illite, page-like kaolinite. The content of illite / smectite interstratified minerals is less. Different kinds of clay minerals can lead to different kinds of reservoir damage. The chlorite mainly causes acid sensitivity. The illite and kaolinite are the main source of speed-sensitivity damage. The illite / smectite interstratified minerals could easily lead to water-sensitivity damage. Through the above studies, this paper can provide foundation of geological basis for rational development.

Research on Calculating Rational Well Spacing Based on Classification of Dynamic Reserve in Ultra-low Permeability Sandstone Reservoirs

Research on Calculating Rational Well Spacing Based on Classification of Dynamic Reserve in Ultra-low Permeability Sandstone Reservoirs

The distribution rule and seepage effect of the fractures in the ultra-low permeability sandstone reservoir in east Gansu Province, Ordos Basin

To study the impact of the fractures on development in the ultra-low permeability sandstone reservoir of the Yangchang Formation of the Upper Triassic in the Ordos Basin, data on outcrops, cores, slices, well logging and experiments are utilized to analyze the cause of the formation of the fractures, their distribution rules and the control factors and discuss the seepage flow effect of the fractures. In the studied area developed chiefly high-angle tectonic fractures and horizontal bedding fractures, intergranular fractures and grain boundary fractures as well. Grain boundary fractures and intragranular fractures serve as vital channels linking intragranular pores and intergranular solution pores in the reservoir matrix, thus providing a good connectivity between the pores in the ultra-low permeability sandstone reservoir. The formation of fractures and their distribution are influenced by such external and internal factors as the palaeo-tectonic stress field, the reservoir lithological character, the thickness of the rock layer and the anisotropy of a layer. The present-day stress field influences the preservative state of fractures and their seepage flow effect. Under the tectonic effect of both the Yanshan and Himalayan periods, in this region four sets of fractures are distributed, respectively assuming the NE-SW, NW-SE, nearly E-W and nearly S-N orientations, but, due to the effect of the rock anisotropy of the rock formation, in some part of it two groups of nearly orthogonal fractures are chiefly distributed. Under the effect of the present-day stress field, the fractures that assume the NE-SW direction have a good connectivity, big apertures, a high permeability and a minimum starting pressure, all of which are main advantages of the seepage fractures in this region. With the development of oilfields, the permeability of the fractures of different directions will have a dynamic change.