Surface Seismic While Drilling - A new method for relief well drilling
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The first relief wells emerged out of necessity, and have since been shaped by several disastrous events throughout the 20th and 21th century. The modern relief well shows little resemblance to the first relief wells that were drilled in the early 20th century. Today?s relief wells are drilled to directly intersect a blowing wellbore at depths of several thousand meters, and often with a diameter of less than one foot. At these depths, conventional positioning surveying tools can yield horizontal errors approaching 100 meters, and therefore cannot offer the accuracy required to facilitate direct intersection between the two wells. Because of this, special survey tools have been developed to home in on the steel tubular in the blowing wellbore. Ranging ToolsTwo types of tools generally exist. Passive magnetostatic tools use conventional magnetic survey tools integrated into the bottomhole assembly to measure the remanent magnetism present in steel, and have a range of 15 meters. Active electromagnetic tools apply an alternating (AC) current to the blowing tubular to inducing a magnetic field around the blowing wellbore. This current can either be applied directly to the casing of the blowing well at the surface, or be injected into the subsurface formations through an injector electrode integrated into the tool. Under ideal conditions, a downhole sensor package can detect the magnetic field at a distance of 60 meters. This survey method traditionally requires a specialized electromagnetic survey string to be run into the hole. Wellbore Positioning SurveysTo be able to utilize magnetic ranging instruments to home in on the casing of the blowing well, conventional surveys must be able to safely position the relief well within the maximum ranging distance from the blowing well. Considering uncertainty in wellbore position of both wells, this can be a challenge. Typical relief well strategies involves drilling towards the blowing well, using conventional magnetic positioning surveys integrated into the measurements while drilling (MWD) package. Before the large uncertainty makes premature interception possible, a gyro-survey is performed to reduce the positioning uncertainty. Once the wells are within the ranging distance from each other, the homing in process can start. Trajectory RequirementsGenerally it is desired to intersect the blowing well at a relatively narrow angle of between 3 and 15º. Because of the surface conditions at the blowing well site, the relief well is often spudded more than one kilometer away. Because of these two conditions, the relief well is drilled at a high angle to approach the blowing well, and is dropped down to near parallel before intersection is made. To increase the accuracy of the homing-in tools, a triangulation approach is often utilized. This is performed by drilling the relief well past the blowing well, at a relative distance of around 10 meters, before it drops down and is drilled parallel to the target well. This technique can reduces the uncertainty of the homing-in measurements to below one meter. Once the wells are at this distance from each other, the pressure drawdown will most likely cause the formation between the wells to collapse, creating a direct hydraulic communication between the wells. Depth of IntersectionBecause of the inherent nature of the surveying techniques utilized to home in on the target, steel must be present in the blowing wellbore. Most often this means that the last set casing shoe will be the deepest point possible to intersect. This means that if an openhole section exists below the casing shoe, this cannot be utilized during the bottom kill operation. Surface Seismic While DrillingJohansen et al. (2013) suggested using repeated Surface Seismic While Drilling (SSWD) to measure the relative distance between the two wells in real time, as the relief well is being drilled. This is performed by acquiring seismic reference data before drilling is initiated, then the seismic survey is repeated continuously as the bit propagates down into the earth. If the reference seismic data is subtracted from the newly acquired seismic data, this will yield only the changes in the subsurface. Hence, the true wellpath can be found. This method has the potential of facilitating a direct intersection, regardless of the presence of steel in the blowing wellbore. Simulation ResultsSimulations were performed to evaluate the benefits of a deeper intersection point. The results showed that a casing shoe intersection would require an injection rate of 290 l/s to dynamically kill the blowing well with seawater. If the well were intersected at the bottom, the dynamic killing rate was reduced to 151 l/s, or a reduction of 48%. The pump power requirements are dependent on the injection method, but the minimum pump power was 19759 and 4942hp, for casing shoe- and bottomhole intersection, respectively. This is a total reduction of 75%. The casing shoe pressure during killing was reduced by 21%. When circulating from the bottom, calculations showed that the well could be killed using a high-capacity drilling rig utilizing the mud circulation system and the cement pumps. A casing shoe intersection required several additional pumps. Depending on deck capacity, pumping vessels or an additional rig would have to be mobilized, increasing the cost of the operation and mobilization time.