Case Study: Steam-Injection Step-Rate Tests Run in the Orcutt Oil Field
Authors: Carpenter Chris 
Published: January 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 169513, “Case Study: Steam-Injection Step-Rate Tests Run in the Shallow Low-Permeability Diatomite Formation, Orcutt Oil Field, Careaga Lease, Santa Barbara County, California,” by Ramon Elias and Ian Marquardt, Santa Maria Energy, and Mason Medizade, California Polytechnic State University, prepared for the 2014 SPE Western North American and Rocky Mountain Joint Regional Meeting, Denver, 16–18 April. The paper has not been peer reviewed.

The operator has initiated a cyclic-steam- stimulation (CSS) project in the Opal A diatomite of the Sisquoc formation on the Careaga lease in the Orcutt oil field in Santa Barbara County, California. The operator has received entitlement to proceed with an expansion consisting of 110 additional new wells. This paper discusses steam-injection step-rate tests (SRTs) for this asset.

Introduction

The target zone contains high oil content ranging from 1,800 to 3,000 bbl/acre-ft in massive intervals with 200- to 700-ft thickness at depths of 600 to 1,000 ft and with a permeability of 5 to 15 md. The pilot currently consists of 19 cyclic-steam-injection wells configured in a 4×5 matrix spaced approximately 120 ft apart, producing from an average depth of 925 ft.

A supervisory control and data-acquisition system is used to control and monitor various aspects of field operations, especially the steam- injection-process protocols. Relatively low steam-injection rates of 250 to 450 B/D coldwater equivalent are used with 70% constant- quality steam. Steam-injection pressures and rates have been monitored closely throughout the process since startup in October 2009.

More than 1,000 steam-injection cycles have been completed as of this paper’s writing. For all cycles, the onset of formation parting has been detected less than 0.5% of the time. In all detected events, steam injection was adjusted or halted without loss of steam-injection containment. Steam injection was resumed when it was determined to be safe to do so, and matrix flow was re- established. To this end, two operational procedures exist for analyzing data as part of the operator’s efforts to improve steam-injection practices further and to improve understanding of the mechanisms leading to oil production. One is taken from common waterflood surveillance practices and graphically displays changes in flow resistance during injection, aimed at indicating if formation parting or a loss of steam confinement is initiating. The other is analysis of the pressure falloff observed during the soak period between injection and production, a period intended to allow pressures and temperatures to dissipate away from the vicinity of the well and into the producing zone.

General SRT Approach

The objective of an SRT is to determine the pressure at which resistance to flow within the rock matrix suddenly decreases as the formation is subjected to increasingly higher fluid-injection rates. That point is said to be detectable when the injection pressure exceeds formation-parting pressure (FPP), which creates increased fluid conduction. This effect produces a reduction in the slope of the pressure-vs.-flow-rate curve. The values of pressure and flow rate corresponding to the point where the two lines intersect in the pressure-vs.-flow-rate curve are called the FPP and formation-parting rate (FPR), respectively (see Fig. 1 in the complete paper). The FPR can be unique to the injection fluid that is used.

In general, an SRT consists of a series of constant-rate injections with rates increasing from low to high in a step-wise fashion following a period of reservoir stabilization. Each constant-rate step is normally run over approximately equal lengths of time.

A discussion of SRT analysis by use of Hall’s method is provided in the complete paper.