Aging Sour Gas Field Successfully Plugged and Abandoned Using Novel Placement Techniques and Intelligent Cementing Fluids - A Thomasville Field Case History
Authors: Glasscock J. M. , Peacock S. , Landry G. , Moriyama A. , Hernandez O. 
Published: October 2018

Since the 1970s, 12 deep vertical gas wells in the Thomasville area in Mississippi, USA have been producing high volumes of sour gas (3 to 21 MMcf/D per well). This production decreased to unsustainable levels, which required the field to be abandoned. Abandonment presented a rare combination of challenges including high temperature (&gt;400°F), high sour gas and CO2 (more than 40% H2S and up to 9% CO2), depleted formation (0.1 psi/ft), scale buildup, unique well geometry, true vertical depth ranging from 20,300 ft to 23,600 ft, and nearby residential areas. A combination of special operating procedures, intelligent self-healing cementing slurries, and novel placement techniques enabled the wells to be successfully abandoned with layers of contingency to prevent a catastrophic environmental release.

The plug and abandon operations were divided into two phases. Phase 1 was rig-less operations to kill the well and isolate the formation by using an engineered ultralightweight 9.0-lbm/gal cement slurry that provides greater corrosion protection compared to a normal cement slurry. Novel placement techniques were used to place the engineered slurry across the production perforations and open hole to isolate both the tubulars and annuli. Advanced hydraulic simulations were run to model the complex placement. A traditional drilling rig was moved in for phase 2 of the operations. In phase 2, intelligent cement plugs, which included flexible and self-healing properties, were placed to add greater zonal isolation assurance accounting for unknown well conditions for the long-term abandonment of the well. Cement plugs were verified with robust negative and positive pressure tests.

It was determined that an ultralightweight slurry could be placed with a 0.1-psi/ft fracture gradient using nitrogen displacement, optimized slurry volume, and variable choke to regulate pressure on the backside to isolate the wellbore. Displacing with nitrogen proved to be challenging, and the many lessons learned will be documented in this paper. All 12 of the producing wells, along with 5 disposal wells in this field, were successfully killed and plugged. To date, none of the wells are showing pressure. This paper will review the challenges faced with designing a successful P&A program in this Thomasville area. Both phases of the final operations will be presented, and lessons learned along the way will be discussed.

These complex well conditions were overcome through sound designs in operational planning, cement slurry optimization, placement techniques, and isolation testing methods. The primary plug proved to be effective at being placed in a depleted environment and at ultrahigh temperatures while taking into account corrosion protection. The intelligent cement slurry offered long-term barrier assurance through both failure prevention and self-repair. The long-term solution outlined in this paper is key to preventing a catastrophic environmental release. The innovative placement techniques, contingencies taken, and lessons learned during the campaign will be useful to other technologists in other fields faced with similar conditions.