Optimization Techniques Research On Different Well Types And Well Patterns For Ultra-low And Super-low Permeability Reservoirs Of Changqing Oilfield

Ultra-low and super-low permeability reservoirs in Changqing Oilfield had abundant oil geological reserves, which had become an important part of reservoirs with stable and increased production in the oilfield. Exact partition of reservoir layers and rational well arrangement were essential for successful oilfield development. On the basis of three typical reservoirs as research objects, physical experimental simulations, reservoir engineering and reservoir numerical simulation methods were synthetically used to systematically research the development boundary and policy for the combined production of multi-layered reservoirs, seepage mechanism of thick pay zones and optimization methods for the vertical well patterns and horizontal well patterns. Meanwhile, the intrinsic law of layers partition was revealed, the seepage mechanism of thick pay zones was explored, the influences of different well types and well patterns on the development were also dissected and finally a novel well pattern optimization method considering the nonlinear seepage features was put forward. The main research results were as follows:1. With the adoption of large-scale slab model made of ultra-low permeability and super-low permeability sandstone outcrops, parallel displacement simulation experiments were carried out for the first time. The percolation characteristic of commingling production was investigated and the influence of permeability ratio on the production development was discussed from three aspects:nonlinear flow law, output feature and flow field distribution. Permeability ratio, layer number, average permeability and oil viscosity all affected the commingling production of multi-layered reservoir under waterflooding. According to the influence of each factor, one parameter named "quad-element synthetic influence factor" was proposed, with which the development limits for commingling production could be evaluated.2. With the method of reservoir engineering being used and the nonlinear flow features of reservoir oil being taken into account, a mathematical model for multi-layered reservoirs commingling production was constructed, the formula of waterflooding front edge for different stratums was deduced and an example was calculated and analyzed. With a numerical simulation approach being adopted, the influences of internal rhythmic features of thick oil pay, perforation locations and open degree upon the development were also discussed.3. By utilizing the nonlinear numerical simulation software and considering the reasonable matching between the well pattern system and the fracture system, this paper optimized the well pattern, well spacing and fracture parameters of ultra-low and super-low permeability reservoirs. The results demonstrated that:if there were few natural fractures, square inverted-nine well pattern would be suitable for the reservoir; if there were some natural fractures, square inverted-nine well pattern could be employed in the initial stage and could be modified to rectangular pattern in the later stage; if there were enormous natural fractures, rectangular pattern would the best. The optimal well spacing would change along with the alteration of permeability grade and anisotropy degree. Regard to the change of permeability, the row spacing was more sensitive compared with the well spacing; as to the change of anisotropy degree, the well spacing was more sensitive.4. With respect to specific well pattern and well spacing, production wells and injection wells both had optimal penetration ratio and flow conductivity. Compared with production wells, short fractures with high conductivity were more suitable for injection wells. Square inverted-nine well pattern had a best infill adjustment plan, which symmetrically added four wells along two sides of previous well rows and transformed the corner production wells to injection wells, resulting in a new small square inverted-nine well pattern with 45-degree angel to previous injection line. Square inverted-nine well pattern and diamond inverted-nine well pattern both had an optimal adjustment occasion.5. With the utilization of the nonlinear seepage numerical simulation method, horizontal well length, fracture morphology, fracture number, fracture length and fracture conductivity were all optimized for multi-segment fractured horizontal wells in depletion. On this basis, aiming at vertical-horizontal combined well pattern, well pattern form, horizontal well length, fracture distribution pattern, distance between injection wells, perpendicular distance between injection well and horizontal well were all optimized and the corresponding policy of sustaining production and controlling water cut was also researched. With the combination of equivalent borehole diameter model and multi-well pressure drop principle, considering nonlinear seepage flow characteristic and classical area pattern productivity formula, the productivity formula of combined production method of fractured vertical wells and multi-segment fractured horizontal well was deduced.