The disposal domain concept is arguably accepted as the prevalent storage mechanism during the batch injection of drill cuttings. The disposal domain is best thought of as an elliptical realm surrounding the well. This phenomenon is well documented by field and laboratory observations and has been addressed in previous work. Most studies and field operations have centered around vertical or near vertical wells. Rarely have horizontal wells been designed for conversion to injectors or used for disposal and therefore, the effect of batch injection and the created disposal domain have not been addressed. The current paper will provide an integrated look at the fracturing process that occurred in a horizontal disposal well. The well is located in the Valhall Field, North Sea, offshore Norway. The well has multiple perforated intervals. The work will address a modified disposal domain concept as it applies to horizontal wells. The paper illustrates a history of the fracture nature (geometry and extent) and propagation across the various layers. Comparison of the field pressure history and the simulation results will be addressed. Further studies to address the effect of a workover carried out in the well, plug placement, are currently under way. These modifications will be addressed in a future paper.
Amoco started the Valhall drill cutting disposal project in November 1990 (1, 2) and well operations continue under BP. While some issues that pertain to batch injection in the horizontal well remain the same as those in vertical wells, there are several new complexities and benefits to using the horizontal well for an injection scheme. In representing the dynamic stress field of a horizontal injector, conventional stress field modeling has been found to be inadequate for the following reasons. The shape of a horizontal disposal domain differs from that of a vertical well. In a horizontal injector, the disposal domain is stacked along the well, whereas the disposal domain incases a vertical well. Concerns regarding stress increase and leakoff reduction are still present in this scenario but have taken on new forms due to the horizontal layout of the well. The stress increase per batch is less in a horizontal well due to the larger volume affected and any leakoff damage is mitigated along the horizontal extent.
The first goal of fracture analysis is to determine when and where new fractures are created every time a new batch is injected. The analysis illustrates how the various parameters interact to provide guidance on the fracturing process during batch injection into horizontal wells. These parameters include wellbore hydraulics, frictional pressure drop along the wellbore, increase in net pressure during fracturing (i.e. the excess pressure in the fracture over the far-field, in-situ stress) and the incremental stress change due to more solids in the formation.
Geomechanic analysis is used to assess the increase in stress caused by single and multiple fractures and their effect along the wellbore. Finally, multiple fracture growth along the horizontal well in successive batches is discussed.