Remanding Oil Distribution In Fractured Reservoir With Low Permeability

With development of petroleum industry, unconventional reservoirs, such as low-permeability reservoirs, play an increasing role in the current and future oil and gas business. In China the estimated ultimate recovery (EUR) oil and gas from low-permeability reservoirs is56×108t, while the total EUR oil and gas from all type of reservoirs is just150×108t. It is well-known that the strength and brittle of rock will increase with the compaction intensity, which further control the fracture distribution. Local stress field is another main factor that controls the fracture patterns in low permeability reservoir. Factures in permeability reservoirs plays a role of a double-bladed sword. They can notably increase permeability to provide a pathway for fluid flowing from reservoirs to well location. But unfortunately they can also become an obstacle for residual oil flowing. Thus it is important to study the distribution of residual oil and fractures patterns in low permeability.The Ordos basin, which locates in the mid-west of China, is the second largest sedimentary craton basin. It is a multicycle-sendimentary petroleum-bearing basin and is well-known for its rich oil and gas bearing. Most of reservoirs in this basin exhibit the properties of low porosity and permeability. The Baibao oilfield, which locates in southern part of Shanbei slope in Ordos basin, is a monocline area with high level in the east and low in the west. There is a good structure inheritance during the tectonic evolution. Chang63formation of Bai209is a representative low permeability reservoir, and previous improper development strategies have heavily hindered the maximum oil and gas recovery for this formation.This dissertation addresses the issues for low permeability reservoir development. By integration well logging, reservoir geology, and statistical data, first a workflow is proposed to study the low permeability reservoirs. Then a practical development stratifies is present for low permeability reservoirs, proving the feasibility of our theory applied to Chang63reservoir of Bai209oilfield.The main results and concussions of this thesis are as follow:(1) The rock type of Chang63layer in Yanchang formation from Baibao oilfield is dominated by the fine-silty sandstone. And most of the matrixes are feldspar and quartz while the ingredients of cements are carbonate mineral, siliceous rocks. The reservoir property is low permeability and low porosity.(2) The types of sedimentary facies of Chang63formation of Bai209well in Baibao oilfield are mainly the middle fan of turbidite, It can be further sub-divided into four microfacies:turbidity current, turbidity lobes, turbidity overflow, and semi-deep lake mud. The channels in turbidity serves the main oil reservoirs and spreads from northeast to southwest,(3) The thickness of Chang6331and Chang6322sand formation are respectively between7and11meter, and8and14meter. Both formations have the properties of good continuity and wide distribution in the whole field.(4) The reservoir in studying area is strong heterogeneity. And its porosity and permeability is closely related to the facies type. The heterogeneity of Chang631and Chang63^is relative weak compared to that of Chang633.(5) The different ancient local stress filed control the different fractures patterns. The main fracture directions in Yangchang formation are EW and SN. The fractures type along NE are mainly joints while the fractures in EW are mainly shear fractures.(6) The coefficient of permeability variation in Chang6reservoir form Bai209district is2.8, the permeability contrastis ranges from1000to5700, and the mutaion coefficient are42.33, which is obviously higher than that of standard of strong heterogeneity. This result indicates the fracture is well developed in this formation. The fractures will affect the water-injection rate, which further influence the oil recovery.(7) The distribution of residual oil is controlled by the reservoirs heterogeneity and fracture distribution. It also is affected by the sedimentary microfacies, sand body continuity, the direction of fracture, and fracture connectivity in perpendicularity(8) The residual oil is mainly distributed in two types of reservoir:turbidity channels and edge of turbidite lobes in the study area. The amount of residual oil in the whole field is huge.