Wellbore Stability Study in Bulalo and its Application to Multilateral Well Drilling

Dimabuyu, A.C., Stimac, J.A., Vicedo, R.O., Villafuerte, G.P., and Protacio, J.A.

Unocal Philippines, Inc.

A multilateral (Bul-109) was drilled and completed in 2003 in the Bulalo field to reduce the cost of drilling in the deeper portions of the steam reservoir. An openhole lateral completion, in which the junction of the lateral leg and the original leg was left unsupported by cement or casing, was selected to reduce costs and address the thermal expansion and related stresses of the casing. Since this type of completion entailed risk of formation collapse, wellbore stability studies were employed to determine whether suitable formations existed and establish criteria for selection of the optimum junction depth while drilling.

Wellbore stress models for a variety of pressures and temperatures were conducted by GeoMechanics International (GMI) and Schlumberger to establish minimum formation strength requirements for an unsupported forked junction. These studies required development of a geomechanical model for the field based on UCS tests on a variety of Bulalo rock cores, analysis of drilling induced fractures in FMS logs, leakoff tests, and other wireline log data from previous wells. It was determined that a minimum of 10,000 psi unconfined compressive strength (UCS) was needed for unsupported formations to withstand changes in pressure and temperature throughout the life of the well. FMS, dipole shear, gamma ray and caliper logs were run in an offset well to assess which formation types met the criteria, and how to determine this at minimum cost while drilling.

Aside from the rock strength requirement, other key parameters were considered in selecting the junction location, i.e. absence of washouts, minimal borehole elongation, absence of entry zones, and all these should be within a single continuous rock unit. From a mechanical point of view, the target unit should have a minimum thickness of 40 ft to address thermal expansion of perforated casing liner in the lateral, well bonded casing above and below the window location at least 200 ft of good quality cement above the window location to ensure pressure containment in the annulus and stability during window milling operations, and the window should not be located along across a casing coupling. Since not all reservoir formations met the minimum strength criteria, gamma ray/caliper logs, pressure-while-drilling (PWD) and drilling parameters were used to assess the formations. For the mechanical requirements, a casing collar locator and cement bond log were used to assess these issues.

Bul-109 was successfully drilled and completed as a multi-lateral and is currently one of largeststeam producing wells in the Bulalo field. The parent bore was drilled to a measured depth of 9663’, with the junction targeted at 4160’ to– 4229’ MD and the lateral completed to a depth of 10,500’ MD. Overall this well had one of the lowest costs per unit of steam produced of recently drilled deep wells in Bulalo. With the economic and technical success of the Bulalo- 109 multi-lateral completion it has been recommended that the same drilling, reservoir and geological data sets and analysis process be utilized in future multi-lateral wells.