Abstract
Risk analysis is routinely applied to reserves estimation, but is rarely applied in the production rate forecasting process. Commonly, a history matched model will be used to assess the most likely (P50) production forecast for (re)development, infill drilling or IOR scenario. Approximate methods are then used to assign input values to produce upside (P90) and downside (P10) forecasts. This paper presents methodologies for more rigorous calculation of risked production profiles.
A novel risk analysis application used to assess the economic potential of horizontal well development of the Ekofisk EA formation in the Albuskjell gas-condensate field is presented. The risk analysis involves combining each branch on the decision tree with a numerical reservoir simulator. An areal basecase model is developed to represent the thin, undrained layer that comprised the main target zone. The analysis uses available log, core and production data and generates probabilistic forecasts (P10, P50, P90 and expected value profiles). Thickness, porosity, permeability, initial saturations and horizontal well length are considered to be uncertain variables with the greatest influence on production. Procedures for determining porosity dependent permeability distributions are addressed. Several scenarios are presented including risked production profiles for horizontal wells with lengths from 1000 to 5000 ft. Both conventional platform drilling and coiled tubing drilling are considered as development alternatives.
Introduction
The Albuskjell Field is a gas-condensate accumulation straddling Blocks 1/6 and 2/4 in the southern part of the Norwegian sector of the North Sea. The field, discovered in 1972, came on-stream in 1979. It is divided into two areas, Alpha and Foxtrot, each developed from a separate platform. The Foxtrot wells were temporarily plugged and the platform shut-down in 1990. The Alpha platform is still producing.
The field is located in the Central Graben of the North Sea and is part of the "Greater Ekofisk" area. Figure 1 shows a location map and a top Ekofisk structure map. The Albuskjell structure is an elongated, NW-trending, doubly plunging anticline, about 18 km long and 5 km wide.
The Albuskjell reservoirs consist of reworked chalks of the Ekofisk and Tor formations. These chalks are of a layered/ no-crossflow nature as a result of the alternation of chalk beds with distinct sedimentary facies The main production on Albuskjell originates from the Tor formation which has higher permeabilities. Very little has been produced from the Ekofisk formation because of low porosity and permeability. Five wells have been perforated and stimulated in an attempt to exploit the Ekofisk formation on the Alpha structure (A-01, A-03, A-04, A-15 and A-20), however, only one of them was successful. This is attributed to the low porosity and permeability in the Ekofisk formation. A-01 and A-03 never contributed more than 2% of the total production from these wells. The average porosity in A-01 and A-03 is 24.4% and the water saturation ranges from 36 to 48%. A4)4 was converted to an Ekofisk only producer in May 1991 and initially produced about 4 MMSCF/D. The well currently produces about 1.0 MMSCF/D. A-20 was completed as an Ekofisk only producer in 1980. The well initially produced 8 MMSCF/D but after four months the rates declined to 1.5 MMSCF/D. A-0 and A-20 have average porosity of 28% and water saturation ranges from 31% to 40%.
A-15 is the only well with significant production from the Ekofisk formation. It was completed as an Ekofisk producer in 1988 when a plug was set to isolate the Tor perforations. The initial rate (Ekofisk only) was 17 MMSCF/D. The well was commingled with the Tor formation in Nov. 1992 after nearly four years as an Ekofisk only producer. Today, A-15 produces about 4.7 MMSCF/D from the Ekofisk formation after seven years production from this formation. P. 393
