Abstract

Morichal-01 is a 9° API heavy oil field located in the Venezuelan Orinoco Heavy-Oil Belt. The reservoir comprises unconsolidated, Darcy permeability channel sands at a depth of 3–5000 ft, and contains a STOIIP of 8.7×109 stb. Development has been on-going since 1956 with a 2% recovery factor to date.

A history matched thermal numerical simulation model showed that recovery is unlikely to exceed 4% of STOIIP using conventional production methods, due to viscous fingering by the aquifer and artificial lift related production problems. Using the simulation model, the authors show that in a relatively undeveloped area of the reservoir close to the aquifer, placing horizontal wells to pump off the aquifer influx substantially reduces viscous fingering and sustained dry oil production can be achieved updip, increasing the recovery factor above 10%. Dry oil production is additionally expected to reduce emulsion problems and costs, improve pump efficiency and reduce disposal costs from uncontaminated produced water.

Introduction

The heavy oil reservoirs of the Morichal area are situated at the north-eastern end of the Faja-Orinoco heavy oil belt in SE Venezuela (Figure 1), and cover an area of approximately 240 km2. The Morichal-01 reservoir has a STOIIP of 8.7×109 stb of 9° API oil, of which around 2% has been produced since 1956. An active aquifer has provided pressure support, but at the expense of high water cuts in many wells. An integrated geological and engineering field study was recently completed, and analytical analysis and a history matched thermal numerical simulation model used to evaluate the production methods employed to date. The results, detailed in a previous paper1, concluded that horizontal wells using PCP pumps were artificial lift, rather than reservoir, constrained and that economically viable infill drilling would be unlikely to increase recovery factors above 4% due to viscous fingering by the aquifer; emulsions formed by water breakthrough to the wells were thought to contribute significantly to production and lift problems.

In order to improve these factors, the authors have used the history matched simulation model to investigate the use of horizontal water producers to generate a pseudosteady state effect in the oil leg by pumping off the aquifer, allowing updip dry oil production. A relatively poorly drained area of the reservoir close to the aquifer was used to show the potential of this concept; this area would likely remain largely undrained otherwise due to the risk of very quickly producing excessive water cuts.

Geological model

The Morichal-01 reservoir has a simple monoclinal structure, cut by normal faults, with a dip of 2–3 degrees towards the north; aquifer extension northwards is curtailed by major faulting approximately 10 km into the water leg. The reservoir is comprised of Miocene age Oficina sands, around 4000 feet deep, deposited over an eroded Cretaceous base during a major transgression (Figure 2). Sedimentological description, based on core and log analysis, has defined a retrograding fluvial-deltaic system, with seven stratigraphic intervals (A to G), which has deposited large scale fluvial channels at the base, passing through deltaic environments to marine sedimentation at the top. The major channel sands in the C, E and G intervals have net pay thicknesses up to 250 feet; these extensive good quality sands are ideal for horizontal wells. Inter-layer shales from A down to E, created by small transgressive-regressive cycles, are considered to be sealing throughout the field, whilst localised vertical communication exists between the E, F and G sands, due to erosive contacts and subsequent sand coalescence.

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