Saner Ranch Pilot Test of Fractures Assisted Steamflood Technology
Authors: Stang H.R. , Soni Y. 
Published: December 2007

Recovery of superheavy oils and tars from thin reservoirs poses formidable technical challenges. The steam injectivity achievable with conventional steamflood processes is low and will generally result in low recovery and poor thermal efficiency, In a second pilot test of the fracture-assisted steamflood technology (FAST) process, Conoco Inc. demonstrated that the process can be used to recover efficiently -2 to 3 deg. API [1.093- to 1.052-g/cm3] tar in south Texas. This paper describes the pilot design and operation. Results of a variety of project monitoring and evaluation techniques, such as temperature surveys, reservoir tracer tests, seismic reflection mapping, and postpilot coring, are reviewed. In addition, the results of a numerical simulation model used for a history match of the pilot performance are described. On the basis of these data, conclusions regarding the dominant flow mechanisms are made.

Introduction

A steamflood process, called the FAST process, has been developed that is especially suited to thin reservoirs containing superheavy oil (less than 10 deg. API [1 g/cm3]) or tar. The process, discussed by Britton et al. and described in detail elsewhere, consists of four phases. In the fast phase, each producing well is hydraulically fractured to create a horizontal fracture in the pay zone and then steam-stimulated to heat the formation surrounding the wellbore. Subsequently, the center injection well is hydraulically fractured to create a path between the injection well and each of the producing wells. In the second phase, high-rate steam injection begins immediately after fracturing the injection well. The steam is injected at rates high enough to keep the horizontal fracture open. This enables heat from the injector to reach the heated area surrounding the producing wells quickly, thereby sustaining fairly high production rates. In the third phase. steam injection is continued in the injection well, but an attempt is made to distribute the steam over the entire vertical pay interval by reducing injection rates and increasing fluid withdrawals. The fourth phase involves the injection of water to recover additional tar by means of a hot waterflood. The first test of the FAST process for the recovery of tar was conducted in a 5-acre [2-ha] inverted five-spot pilot pattern located on our Street Ranch lease in pilot pattern located on our Street Ranch lease in Maverick County, TX (Fig. 1). During the 31-month test, a total of 169,040 bbl [26 875 m3] of tar was produced at a steam-to-oil ratio (SOR) of 10.9. (In this paper, SOR is the volume of injected steam measured as condensate at standard temperature and pressure divided by the volume of produced tar corrected to conditions of 60 deg. F [16 deg. C] and atmospheric pressure.) This represented a recovery of 66% of the original tar in place. The second FAST process pilot test described in this paper was conducted at our Saner Ranch lease in paper was conducted at our Saner Ranch lease in Maverick County. The primary objectives of the second test were to test the process in another part of the San Miguel-4 (SM-4) deposit and to improve the SOR. Several surface-facility alternatives, such as testing of a solid fuel combustor for steam generation and treating of the produced water as a source of boiler feedwater, were also evaluated and are discussed elsewhere.

San Miguel Tar Sand Deposit

Reservoir.

The tar-bearing formation, the SM-4. is a sandstone formation containing many irregular limestone stringers. It has a well-developed limestone caprock, and at most locations, it is divided into two equally thick lobes by a dense limestone streak. Heterogeneity within the sandstone is largely the result of the numerous limestone stringers. The SM-4 outcrops just north of the Maverick/Kinney county line and dips gently south-southeast at a slope of 2 deg. [0.03 rad], reaching a depth of 2,500 ft [762 m] at the southern edge of our property (Fig. 1). Gross thickness ranges from 20 to 80 ft [6.1 to 24.4 m]. High porosities from 26 to 30% and permeabilities from 250 to 1,000 porosities from 26 to 30% and permeabilities from 250 to 1,000 md result from clean, uniform sand. The tar provides most of the cementation. Estimates of the original tar in place in the SM-4 sand vary from 2 × 109 to 3 × 109 bbl [320 × 106 to 480 × 106 m3]. Tar saturations range from 20 to 60 % but seldom average greater than 55 % in any continuous 25-ft [7.6-m] interval. The saturation distribution also varies in the two lobes, depending on the location.

Tar Properties.

The south Texas tar is one of the most dense and viscous hydrocarbons in the world. It is postulated that the unique physical and chemical properties of postulated that the unique physical and chemical properties of the tar are the result of severe bacterial degradation. Properties of the Saner pilot tar are summarized in Table 1. Properties of the Saner pilot tar are summarized in Table 1. It has an API gravity of 2 deg. API [1.06 g/cm3] compared to a gravity of 8 to 10 deg. API [1.014 to 1.0 g/cm3] for tar in the Canadian Athabasca deposits and the 10 to 20 deg. API [1.0 to 0.934-g/cm] range for most heavy crudes.

JPT

P. 684