This paper summarizes an integrated field, experiments and computer simulation research program conducted to support a review of the reserves and development plan for the Hamaca Area in the Orinoco Belt (OB), Venezuela. The impact of the foamy oil mechanism on the Hamaca heavy-extra heavy Oil reserves and the importance of understanding this behavior is presented in this paper. The study was conducted in reservoirs with the largest production history (within the OB). The experimental results showed in-situ formation of non-aqueous oil foam with high gas retention, improving oil mobility and leading, therefore, to high well productivity. An experimental recovery factor over 10% was obtained under primary conditions so it was possible to increase oil reserves by approximately 30% over the currently accepted volumes. Experimental results provided input parameters to perform preliminary simulation runs which would allow the modification of well spacing schemes, the generation of a high-probability production profile, and the optimization of artificial lift systems, incorporating larger capacity equipment. Introduction When the exploitation of Hamaca heavy oil reservoirs began, it was assumed that the primary production mechanisms were sand compaction, solution gas drive and thermal effects due to steam cyclic stimulation which improves oil mobility. However, an unexpectedly high cold production lead to research of the drive mechanisms to explain the special production performance. Similar behavior has been observed in the Lloydminster area of Canada. Despite considerable speculation, a number of authors have studied foamy oil behavior and some research has recently been done, yet the mechanism leading to this behavior still remains to be successfully explained. Reservoir properties that were proposed to explain the behavior in the OB and in Canada include unusually high sand permeability and/or critical gas saturation. In Canada, high critical gas saturation is now an accepted property of some reservoirs, the so called Foamy Oil reservoirs. However, a monitoring field program of sand compaction and land subsidence did not show any evidence that this mechanism has taken place in the Hamaca Area. None of these proposed properties and mechanisms are consistent with field observations. On the other hand, numerical models showed high uncertainty when trying to match the reservoir production pressure behavior. As a consequence, a research program was designed as part of an integrated reservoir study. It included a production behavior analysis, an experimental program for fluid/porous media characterization through conventional and nonconventional PVT tests, and solution gas drive experiments at research centers of Venezuela (INTEVEP), Canada (PRI-CMG) and USA (LAB). P. 639

A set of experiments in porous media was performed to determine oil recovery factor during natural depletion for a heavy oil reservoir. Results on "critical or mobile" gas saturation, produced fluid characterization, residual oil saturation, production profile and effective viscosity versus pressure are presented. In order to characterize the ability of the heavy oil to trap the released gas, conventional and non conventional PVT tests were carried out. By comparing the experimental results during differential liberation tests, a gas trapping factor for the oil was obtained. It accounts for the amount of solution gas that has been thermodynamically released but does not form instantaneously a free gas cap. The so called pseudo-bubble pressure was obtained. In this work the hypothesis involved in the "Low Viscosity Model" was also tested. Introduction During the last years it has been a very large discussion about different production mechanisms to explain the production behavior observed in some heavy crude oil reservoirs specially in the Orinoco Belt in Venezuela and Lloydminster in Canada. It has been characterized by high production rates and/or high primary oil recovery, besides a good pressure maintenance. In the past, we have evaluated other drive mechanisms such as compaction, water drive and thermal process. A considerable amount of laboratory and field work has been done to evaluate compaction effects in the area of our interest. The results obtained from that study showed no evidence of subsidence or compaction, therefore it is not likely to be a major production mechanism. On the other hand, no active aquifer has been found either. According to field (well head samples) and laboratory evidence, the heavy and extra heavy Hamaca oil exhibits "foaminess" when produced under solution gas drive. P. 671

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOil recovery greater than 10% OOIP in the Hamaca area of Venezuela's Orinoco heavy oil belt has been forecast based on field observations and experimental data. 1,2 This high recovery can be modeled using drastically different mechanisms: a high critical gas saturation; a very low critical gas saturation combined with a large effective drainage area; or the formation of finely divided free gas that flows with its associated liquid. This paper details an experimental evaluation of solution gas drive recovery and the production mechanism present in the Hamaca region of the Orinoco heavy oil belt.Critical gas saturation data were collected using a sand pack filled with live Hamaca crude oil at reservoir conditions. The sand pack was 2 inches in diameter, 12 inches in length, and had a pore volume of 200 cc. Pressure depletion of the pore fluids was achieved with a sequence of small fluid expansions. A flow rate was picked to mimic the fluid flux at 1000 ft from a vertical well. Pore fluids were allowed to reach phase equilibrium after every expansion. Pore and produced fluid compressibility, produced fluid gas oil ratio and CT scanner derived gas saturations were measured after every expansion.As the pressure declined the produced fluid exhibited a decreasing GOR and a compressibility consistent with liquid while the sand pack fluids showed an increasing compressibility consistent with increasing gas saturation. Free gas production was observed by a simultaneous jump in the produced fluid GOR and compressibility and was also observed with CT scanner images. The CT scanner derived critical gas saturation was found to be 8.3 PV%. A material balance calculation indicated a critical gas saturation of 10.2 PV%.CT scanner images showed free gas accumulating in gas rich regions which were generally surrounded by oil with little free gas. Gas saturation in gas rich regions could reach 70 PV%. As gas saturation increased, gas rich regions began to connect until a network of high gas saturation traversed the length of the sand pack. CT scanner images taken at the beginning of free gas flow were used to visualize this connected network of high gas saturation.The experimental data show that the high solution gas drive recovery is the result of a high critical gas saturation. The data show that the high recovery is not associated to low critical gas saturation and a large drainage area. No finely divided foam or appreciable associated gas was observed in the produced fluid.When a 9 PV% critical gas was combined with oil compressibility and the expected formation compressibility, field performance predictions yielded a 12.5% OOIP oil recovery estimate. SPE 39031Hamaca: Solution Gas Drive Recovery in a Heavy Oil Reservoir, Experimental Results R.

Production Improvement Strategy For Foamy Hamaca Crude Oil: A Field Case. Abstract The purpose of this paper is to show production improvement strategies for Corpoven's MFB-53 extra-heavy and foamy crude oil reservoir, located in the north Hamaca area of the Orinoco Belt in Eastern Venezuela. This study was conducted by a Corpoven-Intevep multidisciplinary team with the support of national and international research centers in the development of a 3D numerical simulation model. The generation of high-probability production profile has been obtained successfully by understanding foamy behavior and using emerging technologies for artificial lift systems such as Electric submersible pumps in horizontal wells. Foamy Hamaca crude oil behavior modeling has permitted the understanding of the high productivity found in this reservoir, especially in horizontal wells. With this, a primary recovery factor greater than 10% and an increase in oil reserves of approximately 30% is possible. These results were included in the development of a 3D numerical simulation model. Matching field experience through 3D Simulation and Geomechanical models provided the definition of new drilling, completion scenarios and an optimum exploitation scheme. From this study, it was figured out that preliminary cold production with horizontal wells should be used prior to secondary recovery processes to achieve a 30% saving in the overall project cost. Introduction The study area is located in the Bare Block northeast of the Hamaca area, within the Orinoco Belt (Fig. 1). Reservoir exploitation (U sands) began in 1982. It has an extension of 165 km2 and an original oil in place (OOIP) of 2500 MMBLS. The largest extra-heavy crude oil recoverable reserves (250 MMBLS) discovered in North Hamaca belong to this reservoir. As of October 1996, 164 wells have been drilled (45 horizontal and 119 vertical), with an accumulated production of 94 MMBLS (3.8% of OOIP). Reservoir performance has been improved through the use of horizontal wells with an initial production of up to 3000 B/D and an average production of 1500 B/D. Reservoir average pressure has decreased only 100 psia from an original pressure of 1220 psia. Due to the high productivity index (7 to 12) of horizontal wells, high volume lifting equipment was required in order to handle the produced fluids. Electric submersible pumps have been used successfully, and 56 MBD of the reservoir production are tied to this lifting method. It is important to mention that Corpoven's heavy/extra heavy oil production forecast is 270 MBD for the year 2005, more than double the current production of 125 MBD. The development of the 3D model backs up the growing potential of Corpoven's expansion program. The main objective of this study is the definition of a production improvement strategy for Foamy Hamaca Crude Oil in Corpoven's MFB-53 reservoir. Scope The scope of the working plan consisted in analyzing, evaluating and defining the following macro activities:Detailed characterization of the reservoir;Establishment of production mechanisms;Development of predictive field behavior capabilities;Optimization of artificial lift methods; andEvaluation of optimum exploitation scenarios. P. 271^