Formation Mechanism Of Fractures Research Articles

Formation Mechanism of Fractures in Tight Sandstone Reservoirs of the Second Member of Xujiahe Formation in the North of Western Sichuan Foreland Basin

Fractures control the distribution of tight sandstone gas reservoirs in the Second Member of Xujiahe Formation in the north of western Sichuan foreland basin. According to the data of filed outcrops, cores, image logs, slice and experimental analysis, we analyzed the genetic types, development characteristics, formation periods and forming mechanism of the fractures in the study area. Tectonic fracture and diagenesis fracture are the two types of fracture in the Second Member of Xujiahe tight sandstones. Between the two types, tectonic fracture is the main. They are mostly shear fractures with three sets of NEE-SWW, NNE-SSW and NW-SE orientations. Structural shear fractures mostly developed under the tectonism of the end of Indosinian, Yanshan and Himalaya stage. The force that formed tectonic fractures came from compression of Longmen Mountain and Micang-Daba Mountain, overpressure formed from deep buried and stress caused by uplift.

Experimental Research on Brittle Shale Failure Caused by Drilling Fluid Activity

The crack formed under the action of circular medium fluid around the wellbore of brittle shale has been a hot issue in recent years. This paper considers the brittle shale micro fracture formation mechanism from the perspective of inhibitive ability of drilling fluid activity. The model of activity-swelling-hydration is set up firstly through revising generalized Usher model, it states the drilling fluid activity range to keep wellbore in osmotic hydration stage with the critical expansion ratio, thus puts forward the concept of activity window and is verified by indoor test. Results show that the different activity of drilling fluid brings difference in rock strength in macro and time sensitive effect for internal structure of rock in micro. This mechanism can provide a more accurate range design for drilling fluid activity and strong guarantee for wellbore stability.

Formation mechanism of extension fractures induced by excavation of a gallery in soft sedimentary rock, Horonobe area, Northern Japan

This paper focuses on the formation mechanism of fractures induced by excavation of a gallery in soft sedimentary rocks in the Horonobe area of Japan. Detailed fracture mapping of the gallery indicates that the fractures consist of both pre-existing shear fractures and excavation damaged zone (EDZ) fractures. EDZ fractures correspond to weak planes associated with bedding planes or transgranular cracks. The EDZ fractures terminate against pre-existing shear fractures. Therefore, even for excavations in soft sedimentary rocks, formation of the EDZ fractures are controlled by pre-existing fractures and earlier weak planes.

三軸伸張試験で形成された来待砂岩中の割れ目の性状と形成メカニズム

三軸伸張試験で形成された来待砂岩中の割れ目の性状と形成メカニズム

Fracture system related to geothermal reservoir based on core samples of slim holes. Example from the Uenotai geothermal field, northern Honshu, Japan

In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fractures were located and identified from the records of lost circulation while drilling. Productive fractures are classified into a dominant high-angle (average: 77°) group, and a less common low-angle (average: 28°) group. The high-angle fractures were formed as extensional hydrothermal fractures in response to fault-value behavior triggered by fluid pressure activation under lithostatic conditions. The low-angle fractures formed as extensional and/or extensional shear fractures under hydrostatic-lithostatic transitional conditions, with subhorizontal σ 1 stress. Both kinds of fractures were accompanied by hydrothermal mineral deposition.

Petroleum Generation and Expulsion in the Constrained Bakken Petroleum System and Its Relevance to the Formation of Fractures: ABSTRACT

The lack of direct and unequivocal evidence for the existence of fractures in the Bakken petroleum system (Williston Basin, U.S.A./Canada) has led to controversial discussions with respect to the magnitude, orientation and frequency of fractures and especially their formation mechanism. Amongst other hypotheses, the creation of overpressuring as a result of high petroleum generation rates was called upon to induce fracturing. On the basis of a comprehensive organic geochemical study incorporating source rock and reservoir data, the present contribution provides evidence for an alternative pressure-controlled fracture formation mechanism: Mass balance calculations performed on Bakken Shale samples covering a maturity spectrum from 0.3% to 1.1% R{sub o} has revealed that the main phase of petroleum formation took place very early during catagenesis (ca. 0.4 - 0.8% R{sub o}) and that its potential has already been realized in those areas of the basin which currently produce from Bakken reservoirs. (1) It has been shown recently that the Bakken petroleum system represents a closed system, i.e. the Bakken sourced oils have not entered the Madison reservoirs. Hence, overall high expulsion efficiencies indicate that Bakken Shale over- and underlying units might represent the principal reservoirs and, most importantly, the strata where overpressuring should bemore » expected. (2) Detailed analysis of Bakken sourced oils which are produced from Bakken reservoirs imply that their bulk and molecular composition may have been altered by significant in-situ thermal alteration which took place after the main phase of expulsion. (3) The hypothesized process of reimpregnation of petroleum from the reservoir back into the source rock system may account for locally occurring high concentrations of solvent extractable organic matter in Bakken Shales.« less