N'Kossa is an offshore field located 60 km west of the coasts of Congo in water depths of 170 m. The field is producing light sweet oil from an Albian age reservoir buried between 3100m and 3400m TVD. In order to access reserves located in the southernmost compartments of the reservoir, Extended Reach Drilling (ERD) was implemented. Six ERD wells have been drilled to date with lateral extensions close to 6500 m leading to total depths sometimes in excess of 8600m. In addition to the challenges pertaining to the drilling itself, the completion also carried its own ones as the formation would require effective acid-stimulation (not only an acid wash) to reach the desired levels of productivity. Stimulation of long intervals, and how to ensure full coverage of treatments is a recurrent topic of debate, several approaches have been discussed in the literature. In the particular case of N'Kossa, this issue was not only rendered difficult by the length of the perforated intervals (up to 1200m) but also derived from the combination of lithology and permeability contrasts existing in the formation: indeed the reservoir is an alternation of rather tight carbonates (with permeabilities as low as 1 mD) and porous sandstones (which permabilities sometimes reach up to 400mD). The contrast in permeability is unfavorable as the high permeability layers are often encountered at the heel of the drains. Finally, the reservoir temperature is 150°C (300°F), leading to the need for retarded acid systems.
Building on the experience acquired from the successive treatments performed on N'Kossa, the methodology and treatment design, fluids and diversion have been continuously evolved. The treatments currently involves two phases: An injectivity initiation is performed via Coiled-Tubing creating an artificial thief zone at the toe of the well; then a massive treatment based on emulsified acid and ball-sealer diversion is bullheaded from a stimulation vessel.
This paper will discuss design considerations and operational aspects of the acid treatments performed of these ERD wells. We will discuss some of the observations made and present lessons learnt from such treatments.
Located 60 Km offshore from the Congolese coast, the N'Kossa field was discovered in 1984 and put on production in 1996. The current installation includes two wellhead platforms, a floating production facility and two hydrocarbons storage floating facilities (one for oil, one for LPG). The water depth varies between 150 m to 300 m in the field. Gas and water injection are used for pressure support in the field. The general set-up is showed on fig 1.
The reservoir is of Albian age (Senji formation) and is located between 3100 m and 3400 m in TVD; it consists in a succession of carbonated layers and sandstones, sometimes silts. The carbonates occur as silto-sandy calcites or and silty dolomites. The calcites are usually of relatively low permeability: 1 to 50 mD and the dolomites have better petrophysical properties with porosities between 15 to 27% and permeabilities between 10 and 100 mD. The natural fracturing is poor, the production is from the matrix porosity. The sandstones are fine to very fine with porosities values between 8 and 20% and permeability between 50 and 200 mD, sometimes up to 400mD. The sandstone mineralogy can vary from relatively clean to quite carbonated as the carbonate content can often vary between 10 to 30%. Fig 2 gives an illustration of poro-perm data. The bottom hole temperature (BHT) encountered on the wells discussed in the article is 150°C.
The hydrocarbon quality encountered in the reservoir varies with the depth, from a light sweet oil at the base of the reservoir to condensate gas at the top.
The need to access the southernmost compartment of the reservoir from the existing facilities lead to the drilling of wells with significant lateral extension (ERD) (see fig 3) and due to the layered nature of the reservoir, the drains usually intercept the layers initially at 70° and then levels-off to horizontal or sub-horizontal (88° up to 92°); faulting can lead the drain to intercept twice some of the flow-units.
As we saw in the previous paragraph, the architecture of the N'Kossa-South wells as ERDs is directly deriving from the important distance between the production platform and the southern compartment of the N'Kossa field. The drain intercepts the reservoir initially with a high slant and lands horizontally with lengths varying from 600 to 1500m.