Reduction of near wellbore flow restrictions utilizing propellant stimulation techniques has resulted in significant increased incremental production in thermal and non-thermal heavy oil wells in the Lloydminster, Canada area. During the last year more than 100 wells, some of them suspended for periods up to 10 years due to lack of production, have been stimulated resulting in a cumulative production increase of approximately 500 m3/D. The propellant stimulation process has been optimized for shallow heavy oil application through the combination of proprietary downhole ultra high speed pressure recorders, and numerical simulation and analysis. The stimulation technique is economic and similar in cost and operation to perforating. A propellant tool is placed across a perforated zone and ignited producing a high amplitude pressure pulse of relatively low frequency. This pulse attenuates over a large area with three potential stimulating mechanisms:
Specific Pressure Loading Rate, As the propellant deflagrates (burns rapidly) a pressure loading rate above the tensile strength of any plugging material in the perforations or flow paths followed by a flush of combustion gasses mitigates drilling and completion damage (the propellant removes/reduces the formation damage caused during perforating by flushing the pore space clogging pulverized sand grains in the perforation crush zone);
Low Frequency Pulse. As the propellant burns a low frequency pulse is generated which disrupts sand arches or bridges in the formation; and
Oil Re-energization. Oil in the immediate wellbore area, which may have been degasified due to a pressure drop at the perforations resulting in a higher viscosity oil or ‘visco-skin effect’, is re-energized by the rapid injection of carbon dioxide and carbon monoxide generated by the propellant.
Case histories of vertical and horizontal wells, including the presentation and analysis of computer simulations and high speed pressure recordings of the stimulation event will be presented.
Early propellant stimulations in heavy oil wells were conducted out of desperation as all other common stimulation practices were not effective in a significant number of wells. Propellant stimulations are very economic (approximately equal to the cost of perforating); therefore the potential for production increase far outweighed the risks. Although the burn characteristics of the standard propellant tools in the first wells were poor, good production results encouraged development of a heavy oil stimulation tool.
The concept of propellant stimulation is not new - the ‘modern’ process was developed over 30 years ago (references in literature to propellant type devices go back to the late 1800's). To date, approximately 7,000 wells have been stimulated utilising this technique, all with varying results. Although previous research has confirmed the existence of the basic phenomena, solid progress in the understanding of the physical processes involved and their interrelationships is recent. Difficulties in achieving tool ‘burn’ at shallow depths with limited hydrostatic pressures, as well as the wide spread belief that this technique would not work in semi- to unconsolidated sands, discouraged use of this stimulation technique in heavy oil sands (Figure 1).
The advancement of computer simulation systems and high speed downhole recording equipment has assisted in the development of a systematic approach and procedure for problem diagnosis, stimulation design, and evaluation. The application of an underboosting technique demonstrated by ITT Research Institute led to an enhanced ignition process where a shock wave produced by detonating cord is used to control ignition and burning of an oxidizer fuel mix. Optimum peak pressures in a heavy oil application are in the range of 40 to 70 MPa and are reached in approximately 1 to 1.5 milliseconds.