In unconventional resources, horizontal wells are drilled in parallel at a spacing distance designed to maximize drainage of the reservoir. Lateral well spacing should be such that the drainage radiuses meet, but do not overlap. If drainage envelopes do not meet, then oil and gas are left stranded in the reservoir. However, due to the limited accuracy of downhole surveying methods, positional errors in wellbore placement often lead to deviations by hundreds of feet from the optimal wellbore position. The purpose of this study is to quantify the impact of such wellbore placement errors on reservoir recovery for different surveying methods.
A recovery simulator web application was developed to approximate the effect of wellbore positional error on reservoir drainage. The application requires input parameters to define the drilling scenario being evaluated. These include lateral wellbore length, lateral well spacing and recovery percentage as a function of the drainage radius. A user selects surveying methods to be compared in the simulation. Using the latest error models of the Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA), the application simulates a large number of wellbores drilled with random errors corresponding to the selected surveying methods. The simulation assesses the expected amount of oil or gas left in the field due to inaccurate wellbore placement. It also provides statistics on the likelihood of wellbore cross-overs and lease line infractions.
Initial results indicate that random errors in wellbore placement lead to hundreds of thousands of dollars in unclaimed hydrocarbons for a typical multi-well pad when using standard Measurement While Drilling (MWD). However, this loss is reduced significantly when applying advanced surveying methods with higher accuracy, such as In-Field Referencing (IFR) and Multi-Station Analysis (MSA). The likelihood of wellbore crossovers and lease line infractions is then also reduced significantly.
Wellbore placement inaccuracy in unconventional plays has not been a major concern until in recent years, when drillers began placing horizontal wells closely spaced together. Modeling positional uncertainty and improving survey accuracy has been driven mostly by drilling professionals in order to mitigate anti-collision risk and keep wellbores within lease lines. However, this study shows that improved wellbore placement has further significant economic benefits by increasing reservoir drainage and providing more accurate data for spacing tests and reservoir models.