Integration of Old and New Measurements to Optimize Redevelopment of the Lower Lagunillas Reservoir of Bloque IV, Lake Maracaibo, Venezuela

The Lower Lagunillas of Bloque IV of the Bachaquero field is a supergiant reservoir that has been in production since 1956. We have carried out a pilot reservoir characterization study in the central part of the field, in which we have integrated all the available data into 3D reservoir simulation models whose purpose is to optimize redevelopment of the area with horizontal wells. An analysis of historical production was undertaken in order to gain an insight into the reservoir dynamics. This analysis indicated the inefficiency of gas injection in providing pressure support to the pilot study area and demonstrated the presence of active aquifer encroachment from the south. Anomalies in production behavior and fluid characteristics indicate both lateral and vertical compartmentalization of the reservoir. We have integrated Fourier Transform Infrared (FTIR) measurements of mineralogy from old cores with a comprehensive logging suite in a new well to re-evaluate older, sparse logging suites. Application of a mineral based log evaluation and high resolution processing of the new logs have led to a significant increase in estimates of oil initially in place in the study area. We have used a novel approach to estimate permeability in all of the study area wells. Combining these new evaluations with a revised geological model enabled us to recognize eleven geological layers throughout the area. Formation pressure measurements confirm that partial barriers to vertical communication exist between most of these layers. Cased hole saturation measurements and historical production data indicate uneven sweep of these layers such that five layers contain bypassed oil that could be recovered by horizontal wells. We have constructed very detailed simulation models that describe the lateral and vertical variation in petrophysical properties of each of these layers. These models have been used to select the optimum locations for horizontal wells and optimize the drilling sequence of these wells and their design. It is estimated that each of these wells could recover 1 to 1.5 million STB within five years. Introduction A pilot reservoir characterization study of the central part of Bloque IV of the Bachaquero field, Venezuela was jointly undertaken by Maraven and Schlumberger with the objectives of increasing both offtake rates and ultimate recovery from this mature field by re-development with strategically located horizontal wells. This paper outlines the way in which old and new data were combined to build predictive reservoir models to guide this strategic infill drilling. The first of the wells recommended by this study was drilled under an integrated service contract in which members of the Maraven and Schlumberger study teams participated in geosteering the horizontal section of the well. The Lower Lagunillas Reservoir The Lower Lagunillas Member constitutes the deepest member of the Miocene age Lagunillas Formation and has an average thickness of some 300 ft in Bloque IV. The reservoirs occur in a faulted synclinal trap between 8,100 ft and 11,400 ft subsea that is bounded to the east by the major Pueblo Viejo Fault and to the west by Fault VLC-70 (Fig. 1). The Lower Lagunillas reservoir of the southwestern part of the Bachaquero field was discovered in 1955. Early estimates suggested that about 2 billion stock-tank barrels (STB) of oil were initially in place in the Lower Lagunillas reservoir of Bloques III and IV. Production commenced from the reservoir in late 1956. Peak production of 215,000 BOPD was achieved in late 1959, from 65 wells (Fig. 2). Gas injection commenced in 1965, after production of 389 MMbbl had already caused significant pressure decline. Oil production in June 1992 was 25,200 BOPD, from 28 producing wells. At this time cumulative recovery was 890 MMbbl [45% of original oil in place (OIIP)], with remaining reserves estimated to be 289 MMbbl (13% of OIIP). Earlier Studies The reservoir has traditionally been subdivided into three units: L, M and N (Fig. 3). In most wells this subdivision was carried out on the basis of recognizing three sand units separated by shales. However, in some wells the M and N Sands are in direct communication.