Drilling Fluid Plays Key Role in Developing the Extreme HTHP, Elgin/Franklin Field

Abstract

Abstract The development of the extreme high-temperature/high pressure (HTHP)Elgin/Franklin fields is an example of extending the technical envelope in a safe and cost-effective manner. This paper outlines the successful role of then-alkane synthetic-base mud (SBM) in the drilling phase of this project. This success was based on the design criteria of the drilling fluid being met inpractice. Introduction The Elgin/Franklin fields are located in the Central Graben area in the UK sector of the North Sea (Fig 1). They extend over blocks 22/30b, 22/30cand 29/5b. Elgin/Franklin is an example of extreme HTHP conditions with the main reservoir, the Franklin sands, exhibiting a pressure of 1100 bars and a static bottom hole temperature (BHT) of 200°C (Fig 2). There have been a total of 10 wells drilled during this project, five in the Elgin field and five in the Franklin field (Table 1). These wells have typically been drilled to a depth of 6000 metres at angles between 0 to 37°(Fig 3) and have reached total depth (TD) and successfully run and cemented the liner 30 days ahead of the P50 time curve. (Tables 2 & 3) The main feature of these wells is the rapid increase in formation pressure through the thin transition zone at the base of the Upper Cretaceous. The correct setting depth of the 9 7/8" casing is critical in order to achieve a high enough leak-off to enable the well to be drilled to TD with a 2.15 sg drilling fluid. Even when the casing point is picked correctly the narrow window between formation pressure and fracture gradient is extremely tight for the drilling fluid to operate within when drilling the 8½" section. Towards this end the n-alkane drilling fluid was designed to meet the criteria necessary to fulfil this challenge. The fluid had to have sufficient rheologyto eliminate barite sag but not to give rise to excessive equivalent circulating densities (ECD's) which would exceed the fracture gradient. Its gel structure had to be adequate for barite support during static periods but not so high that excessive pressures resulted when breaking circulation that couldhave led to formation breakdown. The drilling fluid also had to remain stable even when left at BHT's and bottom hole pressures (BHP's) for over 100 hours without circulation whilst electric logs were run. In practice the drilling fluid achieved these targets. Barite sag was never an issue, gel strengths were never excessive and early instances of wellbore instability could be understood and successfully controlled assisted in no small part by the drilling fluids excellent characteristics. Electric logs were run without the need for time-consuming wiper trips, and the liners were all run and cemented successfully, which was essential to having a well that was "fit for purpose". The fact that the wells remained relatively problem free throughout the drilling phase of the project has resulted in significant operational cost savings. Drilling Fluid Design Philosophy These HTHP wells are very expensive to drill, typically costing £20 – 25million per well when drilled without any problems. Should problems occur while drilling wells of these type, costs can quickly escalate dramatically. There have been instances of such HPHT wells costing in excess of £40 million due to problems and some have even been lost after such significant investment due to insurmountable difficulties. The drilling fluid plays a key role in the success of such wells and must be fit for purpose. On the Elgin/ Franklin project itwas therefore essential that lost time due to hole/drilling fluid instabilitybe minimised. A drilling fluid system was designed and laboratory tested tomeet the criteria known to be essential for the successful drilling of these extreme HTHP wells. The harsh environment presented to the drilling fluid has the potential to radically alter its behaviour relative to that on a conventional well. The effects of any contaminants will be greatly increased as the thermal energy pushes reactions faster and further.