SUMMARY: Directional drilling in fields bedding plane formations has become a challenge to the petroleum industry because of the complexity of its operations. Therefore, drilling geomechanics plays an important role in engineering planning and calculation prior to the construction of a wellbore which makes it possible to determine a possible instability of the rock formation associated to the drilling inclination of wellbore and inclination of preexisting bedding plane. This paper analyzes the effect of the attack of angle of drilling on the geomechanical wellbore stability of formations with weak bedding plane or laminations using a finite element model and the Abaqus® software considering the rock as an isotropic medium. Three analytic tests with different drilling angles were developed, allowing to establish relationship between the attack angle and wellbore stability. Variables such stress, deformation and failure in rock bedding plane during drilling were analyzed. The results show that at higher attack angles the greater the wellbore instability is associated with the presence of the weak bedding plane
During the drilling of hydrocarbon wells various types of rock and lithologic formations are drilled, including among them naturally laminated ones or bedding planes formations. These formations contain natural disorders that are called weak bedding plane, which represent a major operational challenge during drilling due to the characteristics of its surfaces with little or no cohesion between them. This causes a displacement and imminent separation of the surfaces, namely, unstable conditions when being altered mechanically with the cutting bit, generating slides and cavings into the wellbore which subsequently cause operational problems during drilling such as stuck pipe or loss of circulation causing extra costs in wellbore drilling. It is of the utmost importance to analyze the intensity of the instability of the wellbore depending on the direction and angle with which the weak bedding plane are perforated.
SUMMARY: This work presents a methodology for mud losses mechanism evaluation based on geomechanics of fractures. Several and catastrophic mud losses events are continuously experienced during drilling the 8½¨section in Castilla Field in Llanos basin, Colombia. Technologies like Manage Pressure Drilling (MPD), thixotropic fluids, LCM (Lost Control Materials), ECD (equivalent circulating density) management were applied to avoid/manage mud losses but the issues associated to mud losses continue being a major problem causing among others wellbore instability in K1 superior formation due to fluid static column variations. According to the events, wellbore instability becomes the new problem causing hole cleaning issues, tight hole and restrictions tripping drill pipe and 7¨ liner. In image logs were detected several natural fractures both open and partially open. Fracture´s hydraulic conductivity hypothesis was proposed. To better understand the problem an evaluation of critically stressed fracture analysis was conducted by estimation of normal and shear stresses in each fracture plane assuming pressure transmission from the wellbore to the fractures. Geomechanical parameters estimated for each interval in which fractures were identified, entered the analysis as an input. Then, the fracture´s stresses were compared to the rock´s failure envelope assuming no cohesion in the planes. As a result, was figure out a reactivation gradient, which is compared to the pressure losses estimated based on the static column height in wells that experienced mud losses. The main observation is that there exists a fracture reactivation pressure lower than the minimum horizontal stress gradient and close to reservoir´s pressure that if is overcome, mud losses take place.
Fractures are discontinuities that create escape paths for drilling fluids and thereby constitute an important mechanism of lost circulation. Most rocks contain fractures of various sizes from micro cracks at grain level to fractures extending for hundreds of feet in the reservoir. In some reservoirs, fractures provide important pathways for the reservoir fluids. Connectivity of the fracture network is its essential property. In the lost circulation context, it affects how much drilling fluid can be lost. In natural fractured reservoirs, the availability of a connected fracture system is essential for production, but is detrimental for drilling (Lavrov A, 2016).
SUMMARY: The mud losses events in the reservoir phase at Castilla field (Colombia – South America) during drilling campaign of 77 wells in 2015 represented an overrun close to $20 MMUS and an accumulated volume of 160.000 bbls of drilling mud lost in the hole. The main reason of these events are associated to crossing structural lineaments during drilling such as faults and set of natural fractures together with the already depleted reservoir sandstone, which results in a drop of the magnitude of the minimum horizontal stress also known as closure pressure of such natural fractures. As a result of the decreased magnitude of the minimum horizontal stress, the natural fractures are considered to be in a critical state of stress increasing the risk of having mud losses through the opened natural fractures. Ecopetrol’s geomechanics team has led the visualization and conceptualization of such mud losses events from ant-tracking models got from seismic data and plotted on the prospects wells surveys. A total of 18 wells scattered in the field were analyzed using the proposed methodology in this paper showing 78% of effectiveness of the prediction mud losses during drilling.
Based on the analysis of the drilling history of 442 wells drilled from 2005 to 2015 it was possible to point that 17.5% of the time was associated to non-productive time (NPT) and 18% of it was due to mud losses events. In Ecopetrol’s oil fields seismic attributes have never been used before to perform root cause analysis of mud losses events in reservoirs with high fracture density. Detailed seismic structural interpretations have been developed in order to improve the root cause analysis of the mud losses events in the reservoir phase using seismic attributes such as ant-tracking having in mind that these attributes are specific measurements of geometrical, cinematic, dynamic or static features that come from seismic data and are mainly used to quantify the amplitude and geomorphological features seen in seismic data.