A Staged Fracturing Treatment for Multisand Intervals
Authors: Williams B.B. , Nieto G. , Graham H.L. , Leibach R.E. 
Published: December 2007

A fracturing method, devised for effectively stimulating massive formations containing many individual sand lenses, has been used to stimulate about 450 wells completed in Eocene-age reservoirs in Venezuela. The method has made possible the development of otherwise uneconomic low-permeability reservoirs.

Introduction

The producing acreage owned by Creole Petroleum Corp. in the Lake Maracaibo area of Venezuela is situated in the Bolivar Coastal field. The field is approximately 57 miles long from the northern to the southern limit and 52 miles across at its widest point. Creole's leases - about 220,000 acres underlying Lake Maracaibo - and the total area leased before 1971 are shown in Fig. 1. Lake Maracaibo is a body of shallow, brackish water approximately 100 miles long and 70 miles wide. The maximum depth is about 100 ft, and the average depth is about 60 ft. The important reservoir rocks in the Bolivar Coastal field are those deposited during the Miocene, Eocene, and Cretaceous ages. Of these reservoir rocks, only the Eocene formations are of interest in this discussion. The Eocene section (see Fig. 2) consists of about 8,500 ft of alternating well cemented sandstones, siltstones, and shales of fresh, brackish, and shallow marine deposition. The Eocene formations are separated from the overlying Miocene formations by a major unconformity. Sands in the Misoa section of the Eocene are designated as the B-1-X through B-9-X sands. Fault blocks within these sands are termed "reservoirs" and given the designation B-1-X.1, B-1-X.2, etc. The B-1-X through B-5-X reservoirs, which we refer to as the "Upper Eocene Reservoirs," normally have average permeabilities of about 10 md. The total gross thickness of the productive reservoirs is 500 to 600 ft, of which 15 to 20 percent is sand. Because of the relatively low formation permeability and the heterogeneous nature of the reservoir, development could not be justified without stimulation to increase production rates. The fracturing technique used to effectively stimulate the Upper Eocene reservoirs is described in this paper. Included are details of the design and paper. Included are details of the design and execution of the treatment, a comparison of theoretical and observed stimulation ratios, average production data for all treatments, and a comparison of results obtained with different staging techniques.

Reservoir Description

The Upper Eocene reservoirs consist of a series of sand bodies that were deposited in a shallow marine environment and represent transgressive offlap from the northeast. Because of the depositional environment, these sands consist of many lenticular, low-permeability stringers interbedded in massive shale sections. The individual stringers are generally thin and cannot be correlated from well to well. The B-2-X sands are typical of the Upper Eocene reservoirs and will be discussed for illustrative purposes These sands increase in depth from about 3,500 purposes These sands increase in depth from about 3,500 ft at the north end of the reservoir to about 6,500 ft at the south end. As previously stated, reservoir properties are very heterogeneous. properties are very heterogeneous.

JPT

P. 897