Drilling horizontal wells in shallow and poorly consolidated reservoirs in deepwater scenarios involves risky operations due to the narrow mud weight window. Risks include severe drilling fluid losses, wellbore instability, and fault reactivation, which, in the worst case, may connect reservoir to the sea floor. This work presents a case study of risk reduction based on geomechanics, which includes concepts of fault reactivation while drilling, permitted plastified area around the wellbore, and fit-for-purpose data acquisition, which allowed a recalibration of the model and timely changes on the drilling plan. The study started with a full 3D geomechanical characterization, which is an advanced way to determine stress distribution in a field, in particular, along the faults. Based on this study, it was possible to locate and avoid zones of higher risks of losses and fault reactivation, mitigating drilling risks. From the study, it was also possible to identify the main uncertainties of the model, which allowed a fit-for-purpose data acquisition plan. The most important missing information was a calibration point for the minimal horizontal stress in the reservoir. Previous drilling experience in the area and geomechanics modelling were not conclusive about losses mechanisms, and the upper limits for horizontal drilling were also not clear. In addition, borehole instability had been shown to be an issue on offset wells; therefore, lower limits for drilling were unclear too. It was decided to drill a pilot hole down to the reservoir, set a packer in the caprock and perform a series of minifrac tests. The measured minimal horizontal stress in the reservoir was revealed to be lower than initially expected, which implied the need to recalibrate the model and make important adjustments to the drilling plan. The model was recalibrated and the safe mud weight window was found to be even narrower. It was identified that a lower and unprecedented mud weight had to be used in the horizontal section, which was an additional risk. To evaluate this risk, the concept of permitted plastified area around the wellbore was used, and a lower mud weight was selected under a risk analysis manner. Based on the study, drilling risks were mitigated and horizontal drilling was performed successfully, with minimal losses and controlled wellbore collapse.
Drilling in deepwater presents risks associated to fault reactivation, what includes severe drilling fluid losses, connection of different reservoirs and, in the worst case, connection of the reservoir to the sea floor, causing oil and gas seeps. Recent accidents along the Brazilian coast showed that such phenomena are real and associated risks should be evaluated before any well planning. This work presents a method to identify and quantify risks of fault reactivation while drilling. An upper limit on the safe drilling window is proposed to account for these risks. The method also includes tridimensional stress analysis, where maps, slices and cubes with reactivations pressures over the faults are determined, what can be used to reduce risks by changing well positioning, casing points and mud weight plan. The proposed method is based on two main pillars. The full 3D geomechanical characterization of the area, which is done through the 3D-Mechanical Earth Model, a consistent way to determine the stresses around faults; and a proper criterion to determine the potential risk of fault reactivation during drilling. For this, a criterion of slip tendency analysis based on frictional constrains is adopted. The method can be applied under two methodologies. The first one represents the general case, where enhanced fault and in situ stresses characterizations are required. Through this approach, each fault plane is analyzed and more precise values of fault reactivation pressure are obtained. The second one is a simplification of the first one, and considers that all media is potentially faulted, where each potential fault is critically orientated with respect to the far field stress. This approach is recommended for exploratory or semi-exploratory scenarios, where faults are not very well mapped and main stresses directions are unknown. A case study for a planning phase of a drilling campaign on a deepwater field located in Santos Basin, Brazil is presented. Fault reactivation risks were identified and quantified through the proposed method. Safe mud weight windows were determined accounting for fault reactivation pressures, which was valuable during well planning. Recommendations for casing points and mud weight plans could be done including fault reactivation risks. Examples of risk mitigation by avoiding problematic zones and by re-positioning planned wells were presented. Results showed that the proposed method is an important tool to identify, quantify and reduce fault reactivation risks while planning any drilling campaign.
The development of heavy oil (14 API) in ultradeep waters (1550 m), in a reservoir 800m bellow mud line is a technological challenge all over the word. This paper will present the solutions applied successfully to drill, gravel and test two horizontals wells under these extreme conditions. Atlanta field is a post-salt oil field located offshore Brazil, in the Santos Basin, 150 km south of Rio de Janeiro. The water depth has approximately 1550 m, the reservoir sits 800 meters bellow the mudline, has an unconsolidated sandstone with an average 38 % porosity with high permeability and a 14 API oil. This paper presents most of the challenges the operator had to overcome to drill, gravel and test two wells already with great success. Both wells have a casing program composed by 36″/16″/11 7/8″ and an horizontal section of 800m drilled in 9½″. Due to the 14 API oil the artificial lift method chosen was ESP (1500 HP) installed at an inclination of 75 degrees of the well. To install the pump the well profile was built with a slant section of 100 m with maximum DLS of 1 degree/100ft to avoid fatigue during the operation, but also a maximum dogleg of 5 degree/100 ft to avoid damages to the pumps while running into the wellbore. To be able to achieve the constrains imposed by the ESP, from mud line at 1550 m to the top of the reservoir at 2330 m the well profile design had to achieve a 43 inclination during the riserless drilling section of the well, the techniques applied are described in details in the paper. The next challenge was to drill the horizontal section with a very narrow margin from collapse to fracture of the well, all the procedures regarding drilling fluids and drilling techniques are presented. The 800m horizontal section was drilled in 9½″ to allow the use of 6 5/8″ premium screens to increase production. To guarantee a minimum content of sand in the produced oil with the objective of increasing the life of the ESP the well was gravel packed. Due to a small fracture gradient of the previous shoe 11 7/8″ the gravel was performed successfully with a special propant with very low weight, again the details are presented. The wells were tested with two different positions of the pump: one at the bottom of the well, and other at the sea level just above the BOP. The objectives of the test were to compare the performance of the reservoir with the pumps in two different positions, but also to study the flow assurance problems that may occur after programmed or non programmed stops of the future production system. All the results and main conclusions are presented. The excellent results obtained from the drilling, gravel and testing of both wells, which are presented in this paper, confirms the potential of Santos basin’s Atlanta field.
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