Abstract

This paper presents the results of a study that evaluated various recompletion and depletion scenarios for a cyclic-steam operation in the Midway-Sunset field. The field has vertical wells on one-quarter acre well spacing and most wells are completed through the entire 1000 ft thick pay. Currently, the upper half of the reservoir exists as a steam chest with remaining reserves occupying the lower portion of the reservoir. Possible production strategies are recompletion of existing vertical wells into the lower half of the existing oil column and drilling new horizontal infill wells. A model of an 80-well portion of the field was constructed and used to history match the field data. After history matching, a number of development strategies were simulated and compared.

In this paper, we present:

  1. Current field status,

  2. Details of the model and input data,

  3. History match results,

  4. Simulation results of the vertical recompletion and horizontal infill wells,

  5. Comments on field results of a horizontal well drilled lower in the oil column in May 1996.

Introduction

Cyclic steam stimulation is a principal thermal oil recovery method in heavy oil reservoirs. In the process, steam is injected into the reservoir for several days to a few weeks and then allowed a soaking period to dissipate energy and form a heated zone in the vicinity of the wellbore. The length of the soaking period, usually 3 to 14 days, is chosen to maximize the heat transfer to the reservoir but to minimize heat losses to the cap and base rock. After soaking, hot oil and water are produced from the heated zone. The production period can vary from a few months to over a year. When oil production declines to a point where is no longer economical to produce, the whole cycle is repeated. Oil production response to the first cycle is usually higher than subsequent cycles.

The decrease in oil viscosity with increasing temperature is the most important factor for improving the oil production rate by cyclic steaming. Increasing the reservoir temperature by steam injection reduces the oil viscosity and causes the heavy oil to behave more like a lighter oil. The net result is improved oil mobility (ko/uo). As temperature returns to its original state the oil reverts to its original viscosity if no distillation effects are present. The increase in reservoir temperature by cyclic steaming also causes the oil to expand and become less dense. The heated oil is then pushed into the well, driven by reservoir pressure or by gravity drainage, and is produced. For thick reservoirs with low pressure, gravity drainage is the dominant production mechanism. Gravity drainage is also responsible for exceptional cyclic responses in steeply dipping reservoirs containing low gravity crudes.

The Midway-Sunset field is located in the southwest portion of the San Joaquin Basin in southern California (Fig. 1). It was discovered in the early 1890's, with the first commercial production reported in 1901. It is classified as a "super giant", with ultimate reserves of 2.75 billion barrels. The majority of the field's production and reserves comes from the Upper Miocene turbidite sand reservoirs. These sands were deposited in a series of interfingering submarine fans, and in some areas accumulated up to 1,500 feet in thickness. Due to the low gravity (14 API) and high viscosity (2,300 cp. at reservoir temperature of 95 F) oil, thermal recovery techniques such as cyclic steam, steam drive and in situ combustion have been successfully employed to produce oil from this field. The high efficiency of thermal recovery techniques has resulted in a recovery factor of 40% to date, while current field-wide production, at approximately 45,000 bopd is still significant.

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