Isochronal Testing of a Naturally Fractured Gas Reservoir:A Case History
Authors: Aguilera Roberto , Au Anthony D. , Franks Larry N. 
Published: March 2010

Isochronal, modified isochronal, and LIT testing are used routinely as a basis to forecast deliverability of gas reservoirs. This paper shows an application of this technique to the naturally fractured Palm Valley gas field in central Australia. Results are corroborated with the use of a dual-porosity numerical simulator.

The initial isochronal tests of four wells are presented together with the deliverability equations. This is followed by a comparison with 40 months of production history. The data show, that values of C in the Rawlins and Schellardt equation have been declining continuously for all four wells throughout the 40 months of production history.

The conclusion reached is that conventional isochronal test analysis and a constant value of C are not reliable to forecast deliverability of gas wells in naturally fractured reservoirs. Use of this approach can easily lead to optimistic forecasts.

Introduction

The Palm Valley Gas Field is situated in the central-northern Amadeus Basin, Northern Territory, Australia (Figure I), approximately 120 km southwest of Alice Springs. Geologically the structure has been defined as an arcuate anticline mapped from surface expression and seismic data (Figure 2). The amount of seismic data, however, is very limited due to the very rough surface topography. Although the western and eastern plunges are thus poorly defined, the anticline axis can be traced for over 40km.

A stratigraphic column of the Amadeus Basin including distribution of gas reservoirs in the Palm Valley Field is presented on Figure 3. Gas has been found in the lower Stairway sandstone, the basal Horn Valley siltstone, and the Pacoota sandstone, all within the Larapinta Group of Ordovician Age. These reservoir units can be summarized as follows:

The Stairway Sandstone varies in thickness between 295 and 310 m (969 and 1018 ft.). It is a shallow, intertidal marine sequence of sandstones siltstones and shales. A basal quartizitic unit of approximately 40 m (130 fl.). It is occasionally pyritised and fractured, and has associated good gas production.

FIGURE 1: Location map of the Palm Valley gas field. Illustrations available in full paper.

FIGURE 2: Palm Valley field - Top Pacoota structure showing location of gas pipelines. Illustrations available in full paper.

FIGURE 3: Stratigraphic column of the Amadeus Basin including distribution of gas reservoirs in the Palm Valley field. Illustrations available in full paper.

The sandstones, also termed orthoquartzites, are generally very line to fine, and once in a while medium to coarse grained. The siltstones are partially sandy, dark grey to black, micromicaceous-hard and generally silteified. The shales are dark brittle and hard.

The horn Valley Siltstone consists of siltstone and shale with thin interbeds of limestone and dolomite. The formation was deposited on a shallow marine shelf and ranges in thickness between 100 and 114 m (330 and 375 ft). A limestone and dolomite layer 3m (10,15 ft) thick at the base of the formation is gas productive in Palm Valley 1. The Pacoota Sandstone, the main gas bearing reservoir, lies conformably below the Horn Valley Siltstone. It consists of nearshore, marine, interbedded sandstones, siltstones, and shales.