The Tharjath 11 well, drilled in Block 5A in Sudan (Fig. 1), marked the first utilization of an openhole completion technique in East Africa, where cased and perforated completions have historically been the norm. The friable sands of Block 5A dictated that screens be placed across the producing zone in order to prevent sand production. The reservoir section of Tharjath TJ 11 was planned to be drilled as a 400-m horizontal 8 ½-in. hole through the unconsolidated Bentiu SS1 and SS2 sands. For this development to be economical, production from this well needed to be maximized.
In order to ensure the fluids selected to drill and complete the well would not impair productivity, an extensive suite of laboratory work was carried out, including detailed core analysis, fluid return permeability, and screen flow-through testing. As invert emulsion-based drilling fluids can not be used in Sudan, a low-solids, water-based reservoir drill-in fluid (RDF) was selected. Preliminary formation damage testing suggested that a chemical 'breaker' was required to degrade the filter cake sufficiently to facilitate production with a low drawdown pressure. In order to prevent completion assembly corrosion and remove the potential for formation damage related to corrosion by-products, an enzyme breaker treatment was tested and ultimately approved for use.
The 400-m horizontal reservoir section was subsequently drilled with an engineered reservoir drill-in fluid, and the well completed within the drilling schedule. Well tests indicate that the well had a zero skin, and production is in excess of expectations.
Operators are making increasing use of horizontal drilling techniques in field developments to maximize well productivity, access reserves or reduce water coning by reduced drawdown.1,2 The benefits of horizontal wells can only be fully realised if all well sections are flowing without significant wellbore damage. There is little history of horizontal wells being drilled in East Africa, and specifically in Sudan where the first horizontal well (Heglig 31) was drilled in February 2004.