Description:
Behaviour of poorly cemented formations in case of drilling a vertical exploration borehole will be studied to achieve an in-depth understanding of borehole stability problem. Analysis of the granular formation behaviour has a significant importance in identifying stability issues, designing adequate borehole supports and choosing an efficient drilling method. This paper presents numerical investigations on the behaviour of poorly cemented formations in the vicinity of an unsupported vertical cylindrical borehole. Due to poor cementation and therefore granular behaviour of these formations, Discrete Element Method (DEM) was identified as being well suited for developing realistic models. To conduct the numerical studies a cube of 8m3 made up of spherical particles with diameters ranging from 5mm to 70mm was constructed and analysed in three-dimensional Particle Flow Code (PFC 3D). It is a discontinuum code used in analysis of the granular materials where the interaction of discrete grains is considered. A cylindrical opening with the diameter of 0.3m runs along the central vertical axis of the cube simulating the presence of a borehole. The stresses applied to the cube simulate the underground conditions around an exploration borehole at the depth of 80m. The effects of in situ stresses around the borehole, strength of particle bonding and fluid flow pressure on the stability of the formation around the borehole have been investigated. It has been shown that the development of in situ stresses in the ground due to drilling a borehole results in the formation of a plastic zone around that borehole. When there is lack of sufficient bonding between the sand grains, the interaction between them results in their movement towards the borehole opening and thus eventuates the collapse of the borehole wall. Furthermore, the presence of high pressure water flow expedites the process of the borehole collapse. •We simulate the borehole in poorly cemented formation by discrete element method.•Borehole stability has been investigated under in‐situ underground stresses.•We study the effect of different bondings between particles on borehole stability.•The effect of confined aquifer on borehole instability has been studied.
Description:
Hydrocarbon production and development of carbonate reservoirs mainly depend on fractures that cross wells. Therefore, quantification of their characteristics has a crucial role in reservoir management. In this research, bedding planes, faults and different types of fractures of a Cenomanian-Turonian carbonate reservoir in the Persian Gulf were studied. Image logs including the oil-based mud image (OBMI) and the ultrasonic borehole image (UBI) were used. The results from conventional petrophysical log suites were compared to the image logs results. Based on the image logs, structural characteristics of bedding planes were reconstructed by estimating their strike, dip, dip direction and layer thickness. Fracture types and their distribution and the geometry in the well were determined. Also, fault zone geometry and type of faults were evaluated. The results indicate that majority of the bedding planes have a N58W strike with an average dip of 18° towards NE. Furthermore, both tensile and shear fractures were distinguished, although shear fractures dominated and the 328/36SW and 29/45NW were found to be the prevalent orientation of fractures. Based on petrophysical logs analyses, four facies groups are distinguished and two statistical relationships are proposed for estimation of discontinuities. The proposed index for evaluation of fractures using petrophysical logs shows good performance and it could be used for wells when no image log is available. Especially in offshore reservoirs, lack of outcrop and unavailability of image logs of old wells lead to unknown fractures characteristics which will be addressed by the suggested equations.