Drilling and completion of horizontal wells in any formation is often challenging. However, typical challenges are magnified in the diatomaceous Opal A Monterey Formation in the Belridge Field of Central California. Soft formations tend to be enlarged during wiper trips, while cuttings-bed deposition, lost circulation, and gas migration can complicate cementing. Because of these challenges, horizontal-well cement jobs in this field may not have been capable of providing effective containment of proppant fracture treatments within the target zones. Each well has several subzones, and each subzone is fractured separately. However, due to lack of containment, production results have been difficult to interpret because determining which fracture stage is producing the fluids is impossible. This paper outlines the methods used to greatly improve zonal isolation between subzones.

Reducing the number of wiper trips and improved drilling fluid, directional drilling techniques, and tools helped provide improved hole geometry without cuttings bed accumulation. Fully automated foamed cementing technology was applied to help achieve complete zonal isolation and full cement returns to the surface. Cement jobs were evaluated with special logging techniques and leading-edge cement-bond logging tools with nontraditional interpretation algorithms. Finally, tracer materials were incorporated in proppant fracture treatments to verify that fractures were maintained in the appropriate zones. Detailed descriptions and actual job data are provided to document the significant improvements in drilling horizontal wells in this challenging field.


Drilling horizontal wells in the Monterey formation in central California, outside of Bakersfield, allows operators to recover oil from the margins of a structure that would be uneconomic to complete with vertical wells. Between 1995 and 2001, seventeen Diatomite horizontal wells that required cemented and fractured completions were drilled. The quality of the cement jobs before 2001 was generally unsatisfactory. These wells were drilled in the Opal A and Opal CT segments of the Monterey formation (Fig. 1). The Opal A wells are drilled in the more shallow 700- to 900-ft (213- to 274-m) total vertical depth (TVD) range. The Opal CT wells are drilled in the deeper 2,300- to 2,600-ft (701- to 792-m) TVD range (Fig. 2).

Hole enlargement and lack of cuttings removal caused channeling in the completion interval in some of the wells. Fracture treatments did not always stay in the desired treatment zone, indicating that mud and solids displacement in the annulus was incomplete. The poor cement jobs in these wells were most likely caused by the following:

  • Hole enlargement

  • Cuttings bed

  • Inadequate displacement of mud and drill cuttings

For the drilling of four horizontal West Flank Pilot wells and three horizontal Opal CT Nose wells in 2001, obtaining zonal isolation across the horizontal completion interval to isolate fracture stages was important. An attempt was made to address all the potential causes of poor cement jobs while designing the 2001 horizontal wells.


Belridge field was discovered in 1911, but production from the fractured shales of the Monterey Formation did not become significant until advanced sand-fracturing techniques became available in the late 1970s.

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