S. Zhang

A numerical study on horizontal hydraulic fracture

A 3D non-linear fluid–solid coupling model for horizontal fracture of vertical well was established with the ABAQUS code. The wellbore, cement casing, perforation, pay layer and barriers were included in the model. Fluid–solid coupling elements were used to describe the behavior of formation stress–seepage flow coupling; pore pressure cohesive elements were employed to simulate the process of fracture initiation and propagation in formation. A typical horizontal fracturing process of a vertical well of Daqing Oilfield, China was simulated with the model. All the concerned parameters in simulation were taken from the field measurements. The simulated bottom-hole pressure evolution is consistent with the data measured from the field. The configurations of the fracture and porous pressure distributions in the fracture are presented and discussed.

Risk Assessment of CO2 Geological Storage and the Calculation of Storage Capacity

Abstract Greenhouse gas emission is becoming serious, and attention has been paid to CO2 geological storage which will not only help to achieve CO2 reduction in air but improve oil recovery. This article provides risk assessment of CO2 geological storage and calculation of storage capacity, which was validated in an oilfield.

Major Challenges for Reservoir Upscaling

Abstract In the past two decades, many upscaling procedures have been proposed. The major methods are power-law average, renormalization technique, pressure-solver method, tenser method, and pseudo-function technique. The common problem of conventional upscaling methods is that they tend to smear out the spatially continuous extremes, such as shale barriers and open fractures. However, experience and previous simulation work in heterogeneous reservoirs have shown that oil recovery (especially water breakthrough oil recovery) mainly depends on the spatial connectivity of the extreme permeability values. Lasseter et al. (1986) proposed that scale-up of properties should be done from the scale of a representative elementary volume (REV), a volume for which the measured property does not change with an increase in scale over a given limit of scale. The notion of REV is physical-model oriented and proposed a criterion for upscaling technique. However, the decisive factor in upscaling is the grid system rather than individual physical point. This article concludes that how to obtain REV for real system, how to evaluate the upscaling results quantitatively, how to treat extremes of permeability, as well as how to perform upscaling for naturally fractured reservoirs and carbonate reservoirs are the remained major challenging problems in this area. It is concluded that a great effort should be made on how to obtain the REV grid, which could best describe the heterogeneity of given reservoir at a given scale. Secondly how to perform the upscaling of all properties based on the REV grid is also a critical aspect and should be considered according to the different extent and pattern of heterogeneity of the original geological model respectively.

The Establishment of a Novel Deliverability Equation of Abnormal Pressure Gas Reservoirs Considering a Variable Threshold Pressure Drop

In abnormal pressure gas reservoirs, a traditional binomial deliverability equation is insufficient to accurately describe the gas filtration theory, but only the trinomial deliverability equation considering non-Darcy flow effect and ripple effect can be reasonable to some extent. When gas reservoirs are characterized by low porosity and permeability, strong heterogeneity, and high water saturation, gas flow, similar to liquid flow, is featured in significant threshold pressure gradient effect. Therefore, a threshold pressure gradient is added to the trinomial deliverability equation, and it is derived that the pressure drop caused by the pressure gradient is a variable related to production but not a constant. Consequently, a novel deliverability equation of abnormal pressure gas reservoirs considering variable threshold pressure drop has been established, providing theoretical guidance for determining reasonable gas production in abnormal pressure gas reservoirs.

A Numerical Study on Interference Between Different Layers for a Layer-By-Layer Hydraulic Fracture Procedure

A three-dimensional (3D) solid-fluid coupling model is proposed with the well-known science and engineering analysis software ABAQUS for reservoirs of multi-thin layered pay zoon with low permeability. The cohesive element based on damage mechanics is used for describing fluid flow through fractures and rock crack propagations. Self-developed subroutines are incorporated into ABAQUS for considering viscosity and leak off coefficient of proppant-laden fluid as well as pressure applied on the proppants in the fracture(s) already constructed in preceding fracture step(s). The layer-by-layer fracturing process of a vertical well in Daqing Oilfield of China and interference between different layers are simulated with the model. The numerical results are well coincident with the corresponding data measured in the field. The effects of layer’s spacing on fracture results are studied.

Modeling of Fishbone Multilateral Well Productivity in Row Well Pattern

The technology of fishbone multilateral wells has been extensively applied worldwide in oilfield development. However there are few published models about fishbone multilateral wells productivity. Based on conformal transformation and mirror image theory, a semianalytical model was derived for horizontal wells and fishbone multilateral wells productivity in row well pattern, and verified through the electrolytical model experiment. The matching result showed there was only 12.15% maximum error.

Research and application of a novel strong acidic clean fracturing fluid

Clean fracturing fluids used in the oil-fracturing field are basically alkaline currently. The insoluble precipitate which produced from the reaction of strong alkaline liquid and reservoir formation will cause serious injury to the reservoirs permeability, and the dirt onthe surface of construction equipment is difficult to remove. The strong acidic clean fracturing fluid described here is a novel clean fracturing fluid for proppant-fracturing. It is a viscoelastic solution formed by surfactant and counter ion salt in acid solution, whichcan work in strong acid condition(pH2). It can carry and transport proppant into the specified formation location effectively in the reservoir formation up to 80°C. It has excellent shear-resistant performance and sand-carrying properties, and erode-inhibiting effectalso. The demulsification rate can reach to 90% and more. The novel acidic clean fracturing fluid will break completely when it have encountered hydrocarbons or water but no permanent damage to the formation was caused. The fracturing fluid has both etching effectand proppant-fracturing effect to calcium sandstone reservoir yield significantly. The novel acidic clean fracturing fluid has been successfully applied to the Daqing Oilfield and got remarkable results. Increasing production reached to 9 times for single well, and thestimulation ratio is 2–4 times to ordinary fractured wells.

Successful Method of Fracturing in Offshore Oil Fields in China

Abstract Fracturing technology has been applied very rarely in offshore oil fields in China. Compared with on land, fracturing in offshore oil fields has its own features. Firstly, the cost of operation and fracturing treatment is higher, so a higher ratio of success and effect is required. Secondly, deviated wells and horizontal wells, which are common in offshore oil fields, increase the risk of fracturing. Finally, being restricted by the limited operation space and equipment on the platform, the scale of fracturing in offshore is also limited. With the exploitation of offshore oil fields, more low-permeability blocks need hydraulic fracturing. On the base of study according to the situation of offshore surrounding some successful field experiences are preceded in Qikou 18-2 oil field in Bohai Sea Bay. We present the fracturing strategy applied in offshore oil fields and the method can afford instruction for offshore fracturing in future.