Drilling in Tyler County, Texas, can expose the Wilcox sands and the Midway shale simultaneously, which can create a pressure differential that can require an extra string of casing. In an effort to reduce flat time and stuck pipe incidence, Ambar Lone Star, with cooperation from Ergon Energy Partners, has tailored a “designer fluid” based on the lithology of the well bore.

The idea behind this design is somewhat based on the paper IADC/SPE 87130, “Drilling Fluids for Wellbore Strengthening,” by M.S. M.W. Alberty, M.R. McLean, H.J. de Jong and K.

The Wilcox formation varies in thickness in the area, but is typically 3,500 to 4,000 ft (1,067.5 to 1,220 m) thick. The formation consists of cross-bedded layers of shale, lignite and sand with intermingled combinations of these layers. Medium to very fine quartz sands make up about 50% of the Wilcox.
Armagost, BP Exploration. The authors’ description of the theory is “to actually allow small fractures to form in the wellbore wall and to hold them open using bridging particles near the fracture opening. The bridge must have low permeability that can provide pressure isolation. Provided the induced fracture is bridged at or close to the wellbore wall, this method creates an increased hoop stress around the well bore, which we refer to as a stress cage effect.”
The two main components of this designer fluid are SureSeal and Star LG. SureSeal is a graphite coated, carbon-based material that allows tightly packed particles to remain in place when differential pressures would normally cause them to dislodge or collapse, a major factor in wellbore strengthening. Star LG is composed of multi-sized particles with multiple components (lignite, carbon black, gilsonite dispersed) coupled in a glycol (PPG) /surfactant based liquid. Star LG is an environmentally safe water-based drilling fluid additive with wide particle size distribution. Star LG also strengthens the well bore.

These products have been used in both oil- and water-based fluids and were chosen because of two factors. The first is an ideal particle size distribution. Star LG can be blended to fit almost any particle size range. The second is because the materials used rate high on the Mohs hardness scale. Hardness helps ensure the integrity of the particle size while it is exposed to downhole conditions over time. This is beneficial as research on “wellbore strengthening” shows a problem with degradation of some of the softer materials being introduced even after only one or two circulations.

Armed with a carefully formulated designer fluid that incorporated the stress cage concept and a formation specific drilling fluids plan, the drilling team proceeded to apply this knowledge to actual wells. First, the fluid loss is reduced to a low level prior to penetration of the Wilcox sands at +/- 5,700 ft (1,738.5 m) to ensure a minimal amount of washout and filtrate invasion into this unconsolidated and often underbalanced environment. Once the Midway Shale becomes exposed at +/-10,300 ft (3,141.5 m), a proprietary blend of sweeps are used to bridge particles across the Wilcox sands and to seal microfractures in the Midway shale as pore pressures begin to rise. Sweeps are continued as the Pecan Gap and Taylor shale are drilled and interval total depth in the top of the Austin Chalk at +/-13,200 ft (4,026 m) is achieved.

In the past 12 months, Ambar Lone Star and Ergon have drilled more than 25 miles (40 km) of hole in Tyler County in 284 drilling days without an incidence of stuck pipe or a failed casing run. Average penetration rates in the intermediate hole section has been 938 ft (286 m) per day.

Comments