Description:
The breakage of bonding between sand particles and their dislodgement from the borehole wall are the main factors resulting in a borehole failure in poorly cemented granular formations. The grain debonding usually precedes the borehole failure and it can be considered as sign that the onset of the borehole collapse ia imminent. Detecting the bonding breakage point and introducing an appropriate failure criterion will play an important role in borehole stability analysis. To study the influence of different factors on the initiation of sand bonding breakage at the borehole wall, a series of laboratory tests was designed and conducted on poorly cemented sand samples.
Description:
The breakage of bonding between sand particles and their dislodgement from the borehole wall are the main factors resulting in a borehole failure in poorly cemented granular formations. The grain debonding usually precedes the borehole failure and it can be considered as sign that the onset of the borehole collapse ia imminent. Detecting the bonding breakage point and introducing an appropriate failure criterion will play an important role in borehole stability analysis. To study the influence of different factors on the initiation of sand bonding breakage at the borehole wall, a series of laboratory tests was designed and conducted on poorly cemented sand samples.
Description:
Considerable stability problems, such as weakening of the rock formation and borehole convergence, can occur when drilling into areas comprising a poorly cemented formation. In this study, a laboratory-scale convergence measuring device (CMD) has been developed based on the local deformation transducer (LDT) concept, and successfully applied in a series of laboratory tests to continuously monitor borehole deformation in synthetic thick-walled hollow cylinder (TWHC) specimens with various cement contents. Displacement measurements obtained from the CMD were calibrated against a digital micrometer. Calibration results showed that the performance of the CMD and the digital micrometer matched with high accuracy. TWHC test results showed that an increase in the cement content resulted in an increase in the deviatoric stress required to initiate a borehole convergence. Moreover, it was observed that the increment in the level of the deviatoric stress required for the borehole convergence initiation decreased, as the confining pressure increased. These findings can help design more appropriate support systems for drilling operations involving exploration boreholes, as well as oil and gas wellbores to improve the drilling performance in poorly cemented formations.
Description:
Achieving an in-depth understanding of the behaviour of low cemented formations such as running sands around a cylindrical borehole is of significant importance in identifying stability problems, designing adequate borehole supports and choosing an efficient drilling method. This paper presents numerical investigations on failure modes of unsupported vertical cylindrical boreholes drilled through weak formations, i.e. low cemented sands where the movement of individual grains is not restricted by sufficient cementation. Distinct element method (DEM) was used for this study. The method enables to simulate sand grains and control normal and shear bonding between them. To simulate actual condition surrounding a borehole at a depth of 80m, a 2m × 2m × 2m cube made up of spherical particles with diameters ranging from 5mm to 70mm was constructed and analysed in the DEM code, PFC3D. The results showed that stress concentration developed in the ground due to the presence of a borehole leads to the formation of a damage zone around that borehole. When there is not sufficient bonding between the sand grains, the interaction between them results in their movement towards the borehole and the eventual collapse of the borehole wall. The effect of fluid flow on stability of boreholes has been studied as well. The cementation between sand grains is mainly caused by the presence of clay particles in the formation. The bonding strength between sand grains is an intrinsic or micro property of the bonding material and is very difficult to measure in laboratory conditions. The results obtained from current research will help to evaluate the effect of particle bonding on the mechanical behaviour of low cemented formations and develop methods to assess borehole stability during and after drilling through such formations.
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.
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:
Adopting an appropriate failure criterion plays a key role in the borehole stability analysis. In this paper the induced stresses on a vertical borehole wall were calculated based on the elastic theory. Then, to predict the stability of a borehole drilled through a poorly cemented sand formation, failure envelopes in different failure criterion domains were derived using the results from a series of precise laboratory tests conducted on solid and hollow cylinder specimens. The mixture used in specimen preparation was designed to simulate the properties of the samples collected from depths up to 200m at a drilling site in South Australia. The hollow cylinder test apparatus was developed by modifying a Hoek triaxial cell. These modifications allowed observing the process of debonding of sand grains from the borehole wall during the test and consequently, acquiring a better understanding on the failure mechanisms of a borehole drilled through poorly cemented sand formations. Three well-known failure criterion domains; Coulomb, Drucker–Prager and Mogi, were considered versus the laboratory test data to investigate their capability to predict the shear failure of a borehole using the data from hollow cylinder tests. The obtained results showed the significance of selecting an appropriate failure domain for predicting the shear failure behavior of poorly cemented sands near the borehole wall. The results also showed that the Coulomb criterion is not well suited for predicting the borehole failure when there is no pressure acting inside the borehole. A failure envelope based on the Mogi domain was developed which can be used for the far-field stress states. The introduced failure envelope allows predicting the stability of a borehole drilled in poorly cemented sands.
Description:
Borehole stability analysis is an important challenge for researchers in the field of geotechnical, mining and petroleum engineering. Several borehole instability problems during or after the completion of drilling, have been reported by a number of exploration companies in Australia. Many of these problems are reported in drilling projects in poorly cemented sand formations at depths of up to 200 m beneath the ground. The sand production problem, as it is known, has also been observed in weakly bonded sandstones where the debonding of sand grains can be triggered by fluid pressure and induced stresses leading to the failure of the sandstone at the borehole wall. The strength of a granular material formation is generated mainly by a natural cementing agent that bonds sand grains together.
Description:
Borehole stability analysis is an important challenge for researchers in the field of geotechnical, mining and petroleum engineering. Several borehole instability problems during or after the completion of drilling, have been reported by a number of exploration companies in Australia. Many of these problems are reported in drilling projects in poorly cemented sand formations at depths of up to 200 m beneath the ground. The sand production problem, as it is known, has also been observed in weakly bonded sandstones where the debonding of sand grains can be triggered by fluid pressure and induced stresses leading to the failure of the sandstone at the borehole wall [1], [2]. The strength of a granular material formation is generated mainly by a natural cementing agent that bonds sand grains together [3].