Ian Palmer,* Amoco, Hans Vaziri, Technical University of Nova Scotia, Mohamad Khodaverdian,* Terra Tek, John McLennan,* TerraTek, K. V. K. Prasad,* Amoco, Paul Edwards,* Amoco, Courtney Brackin, Amoco, Mike Kutas, Amoco, Rhon Fincher, Amoco
Amoco is producing coalbed methane from several hundred wells in both San Juan and Warrior basins. These wells were completed/stimulated in one of two ways:
openhole cavIty completions.
hydraulic fracture stimulations through perforations in casing.
cavity operations are described, and new data from several cavity completions is presented and analyzed. The latest geomechanics modeling of the formation of cavities in coalbeds is presented. The model allows the growth of a cavity as tensile failure occurs, and computes increases in permeability in a stress-relief zone that extends tens of feet from the well. critical parameters are given for the success of cavity completions. A pulse interference analysis is discussed: as well as interwell permeability, this can provide information on stress-dependent permeability. Finally, some wells which were originally cavitated did not perform up to expectation, and have been recavitated with remarkable success - these are also examined.
Amoco has tried several different kinds of hydraulic fracturing treatments. Results of comparisons between foam fracture, slick water fracture, and gel fracture treatments are presented. Statistical comparisons are given for regions outside of the fairway zone in the San Juan Basin. In the Warrior Basin, water fracture treatments with and without sand have been compared. Lastly, foamed water cleanouts, without sand, have been deployed, and their success is reviewed.
In this paper we present new information on completions/stimulations of coalbed methane wells. Specifically. we discuss (1) openhole cavity completions in the fairway (sweet spot) of the San Juan Basin (Colorado and New Mexico - see Figure 1), and (2) fracture stimulations in the San Juan Basin and the Warrior Basin (Alabama).
The openhole cavity completion has been used with tremendous success in the San Juan Basin. Some wells produce in excess of 10 MMCFD from only 3,000 ft depth in the fairway zone (Figure 1). In the cavity operation, a series of injections (or shut-ins) and blowdowns (actually, a controlled blowout) is performed over typically a two-week period. Coal fails and sloughs into the wellbore and is ejected from the well, leading to creation of a cavity (enlarged wellbore). A plastic or shear failure zone is also formed beyond the cavity. and in this region the permeability is changed.
A typical Amoco cavity operation was described previously. Below is an elaboration of certain aspects of cavity operations in the San Juan Basin fairway:
The openhole portion of the well is generally 200–300 ft in height, containing usually more than 50 ft of net coal. The coals are divided into the basal coals, which are usually the more productive, and the upper coals. Normally 7-in. casing is topset above the top coal, and TD is only a couple feet below the bottom coal.
A typical cavity operation entails a sequence of (1) cleanout of the well in the evening using air (1,500–2,200 SCFM) and water (20–100 BPH) injections, followed by (2) flow testing lasting typically four hours, followed by (3) cavity operations (or CST), typically 6-10 surges during the daytime. Before the flow test and CST, the bit is either pulled into the casing shoe or to the surface. The sequence is repeated many times over typically 10–20 days.
All flow tests are conducted through a 3/4 in. choke, typically for four hours. All pressure surgings are conducted by rapidly opening a surface valve, allowing gas and water and coal fines to be expelled through blooie lines to the pit.
The basal seams seem to respond more than the upper seams to the cavity operations, presumably because they are more friable.
It is not uncommon to see 0.5–1 in. pieces of coal come to the surface during cavity operations.
In flow tests in good wells, flows during the cavity operations often decrease with time over 1-4 hrs. This may be the transient effect that is predicted by the cavity modeling (see later in this paper).