Case Study: Using Managed Pressure Drilling and Oil Based Mud to Efficiently Drill an Extremely Thick and Highly Fractured Carbonate Sequence Under an Extremely Thick and Highly Sensitive Laminated Shale
Authors: McGowen Harold E. 
Published: March 2015

Abstract
Managed Pressure Drilling (MPD) was successfully used to exploit the lower cretaceous Buda, Georgetown, Edwards, and Glen Rose formations in Houston and Madison Counties, Texas. The objective was to economically stimulate and commingle all of these zones. A comprehensive solution was developed to combat the principal causes of cost over-runs due to: 1) weak zones in the shallow formations above the productive intervals, 2) wellbore stability issues caused by sensitive shale formations, and 3) lost circulation caused by natural fractures in the productive interval. The operator was faced with several shallow low fracture gradient intervals above an extremely thick and sensitive shale sequence (Dexter Shale) above a 1,400′ pay interval comprised of multiple fractured carbonate horizons. Moreover, the lowermost productive interval had a significantly higher pore pressure and the potential for significant gas deliverability while the shallower productive horizons had a lesser pore pressure and were primarily oil productive zones with associated gas. Problems encountered included excessive shale production, wellbore collapse, BHA sticking, and severe lost circulation. The operator drilled multiple wells overbalanced with various water based mud systems (conventional approach), in certain parts of the field. Success with water-based mud required a great deal of finesse and the operator was unable to achieve consistent, predictable results in the most prolific producing areas. Unexpected cost over-runs occurred and several wellbores were lost and required re-drilling.
Managed Pressure Drilling (MPD) allowed the operator to use Oil Based Mud (OBM) to control the sensitive shale formations while drilling with a mud weight low enough to prevent lost circulation. MPD dramatically reduced OBM losses into extremely high permeability fractures in the productive strata. The operator employed a Rotating Control Head (RCH) along with a gas-buster, choke manifold, fluid handling system, and flare system. This equipment allowed the operator to flow while drilling in the deeper high pressure/high deliverability zones and maintain an “at-balance” bottom-hole pressure in the shallower/lower pressure zones through judicious application of surface backpressure. The oil-based mud eliminated all issues with the sensitive shale formations. A mud weight (including ECD) of about 8.95 PPGE for most of the hole and a baseline LCM load proved sufficient to control lost circulation in most instances. When drilling through intervals with intense natural fracturing, the operator used slugs containing a higher LCM concentration comprised of a broad range of particle sizes. LCM slug materials included cottonseed hulls along with graphite, cellulosic fiber, and CaCO3 to cure/prevent partial lost circulation. In some extreme cases, the operator employed fresh water gel pills (“Reverse Gunk”) to combat total lost circulation. The operator developed a “Mud-Cap” technique to pull the Bottom Hole Assembly (BHA) when Total Depth (TD) had been achieved which improved well control and reduced OBM losses when running and cementing casing. Through producing consistent and economic results over a large number of wells across a large area, the OBM MPD program made the continuous and successful development of the entire field possible.