Wellbore-to-Fracture Communication Problems Pose Challenges in California Diatomite Horizontal Wells

Abstract

Abstract In an increasing number of hydraulic fracturing environments, the petroleum industry has aimed for greater cost-effectiveness through the use of horizontal wells instead of conventional vertical wells. Several recent examples where this strategy has been evaluated include the giant Belridge oilfield where three horizontal wells have been drilled with an oblique orientation of roughly 700 to the preferred fracture orientation (determined by the existing well pattern), and completed with multiple propped fracture stages. In the past, industry has focused much attention on the connection between transverse hydraulic fractures and horizontal wellbores, primarily to avoid bridging screen-outs in the near-wellbore tortuous region. It was assumed that if the proppant could be placed successfully, then the fracture stimulation would be successful. However, in the soft diatomite rocks in the San Joaquin Valley, a different problem has emerged. While premature screen-outs have not been a problem, the complex character of hydraulic fracture initiation in this rock has impeded the creation of a sufficiently conductive connection between the wellbore and the primary propped fracture(s). Integration of several diagnostic techniques (such as net fracture pressure analysis, tiltmeter fracture mapping, Hydraulic Impedance Testing (HIT), post-fracture production logging and video logging) reveals that the poor wellbore-to-fracture connection is due to the complex initiation of multiple hydraulic fractures along the horizontal wellbore, many of which are not effectively connected to the perforations. Despite appropriate far-field propped fracture penetration in the reservoir zones of interest, connection between the reservoir and wellbore is compromised because the main propped transverse fracture can be displaced 10 – 100 ft along the wellbore away from the perforated interval, This breakdown in wellbore-to-reservoir connection can severely damage well productivity, in extreme cases reducing individual stage contributions to zero. Figure 1 shows a conceptual sketch of what the actual fracture geometry induced from a single stage may look like. Note the small longitudinal fracture that runs along the wellbore and the many transverse fractures that leave the wellbore away from the perforated interval. As petroleum production from transversely fractured horizontal wells has been successful in other environments, it is believed that the severity of the problems encountered with these wells was due the specific environment conditions (3D-stress state, rock properties, poor cement quality, etc.). However, these same problems are likely to occur, perhaps to a lesser degree, in many environments. P. 635^