Abstract

Liquefied petroleum gas (LPG) fracturing technology is a new dry fracturing technology developed by the Canadian Gasfrac EnergyServices, first established in 2011 to obtain the annual ‘World Shale Gas Award’ at the World Shale Gas Conference. In 2012, ‘Technology Innovation Award’ was also issued to this technique by the US magazine ‘Exploration and Production’. It was chosen as one of the 25 best inventions in 2013 by the US magazine ‘Times’. This technology uses liquefied propane, butane or a mixture of both as the fracturing medium for reservoir fracturing, which is breakthrough compared with water fracturing.

1. Application of LPG Fracturing Technology

GasFrac company is North America's backbone LPG fracturing service company. So far as December 2013, more than 2300 stimulation jobs have been implemented in more than 700 work sites; stimulation was conducted in more than 75 different formations, including oil, gas reservoir and condensate reservoirs, mainly in western Canada, Texas, Colorado and other areas. Most large-scale operation is to use 800t proppant for fracturing; maximum working pressure is 90MPa; propane treatment rate is 8m3 / min; proppant concentration is 1000kg / m3; maximum operating depth is 4000mTVD; highest fracturing number is 22; the formation temperature is 12 to 150 ℃. It has provided services for 50 international oil and gas companies including Apache, Royal Dutch Shell, Chevron, Husky Energy, Devon Corporation, Murphy Oil and Pennwest Energy.

2. LPG Fracturing Equipment and Technology

2.1 Fracturing Equipment and Ground Process

LPG fracturing system consists of gas gel system, nitrogen closed system, mixed system (gel and proppant), fracturing injection system, remote monitoring system (risk control) and gas recovery system. In addition to conventional fracturing equipments, we need to add LPG fracturing fluid storage, pumping and control equipment. Construction is wholly closed: first gas was liquefied to a gel state, then propant was added for mixing and remote infrared surveillance on fracturing at last.

During the initial flow, desander, two-pipeline furnace, separation equipment and spray equipment were needed. Furnace allows liquid propane completely dry, and all the impurities out, including the fracturing sand already flowback. This ensures that no liquid propane get into the discharge spout or spray tower but directly enter the liquid storage tank to reuse in further fracturing. In subsequent applications, propane recovery system will convert propane back to liquid state, which can be drained out of the well site for burning when it would not be reused.

2.2 Fracturing Process Steps

LPG fracturing process is also different with hydraulic fracturing. On pressure test stage, we need to first add proppant to an airtight container and use nitrogen cycles throughout manifold system to check the system for leaks and cut off contact with air to prevent blasting.

Depending on the phase diagram of a mixture of propane and methane with different ratios and specific well temperature to optimize components of the mixed gas can take advantage of on-site natural gas to reduce costs. When well temperature was too high, it can be mixed with diesel oil, which is more easily converted into a supercritical fluid. On fracturing stage, first inject LPG to propant container and using nitrogen pressure, through pressure and temperature to control thick mixture of LPG fracturing fluid; use pressure pump for the wellbore pressure until the reservoir cracks; open the valve for propant  and pour sand-carrying fluid into the wellbore for fracture-extend sanding - proppant content is generally 50%. When then amount achieves the designed scale, stop the pump, shut. During the injection, with the LPG temperature increase, viscosity decreases, so equipment and materials should be fully prepared to control the construction time and ensure the success rate of fracturing.

In the fracturing fluid flowback stage, first with nitrogen to clean up the ground line, then put spray fracturing fluid flowback. Due to the absorption pressure drop and heat reservoirs, LPG vaporization after fracturing fluid gel breaking, no pumping device, using its own expansion of the pipeline will be able to return to the ground.

When using propane fracturing flowback only spend within the flow of the early 24h, when the output from the wells is 100% propane, you must use the limited flowback equipment to help them put spray, after which the gas relative density of aboutIt fell from 1.5 to about 1.0, then you can be guided to the well fluid gas production plant for processing or sale. In most cases, before the start in the back row 5d, propane content will be less than 10% concentration.

3 Pros & Cons of LPG Fracturing Technology

3.1 Strengths of LPG Fracturing Technology

LPG fracturing fluid substantially avoided water in fracturing process, so water sensitivity, salt sensitivity, wetting reversal and other reservoir damages associated with hydraulic fracturing fluid can be completely eliminated. Viscosity of LPG fracturing fluid can suspend completely, avoiding propant deposition of usual fracturing fluid due to the low viscosity. Compared with hydraulic fracturing fluid, LPG fracturing fluid has a low viscosity, low surface tension, fully compatibility with reservoir fluids and reusability. It effectively overcomes reservoir pollution caused by hydraulic fracturing fluid, low flowback rate and other shortcomings. Meanwhile, LPG fracturing also avoids dealing with flowback hydraulic fracturing fluid after construction, eliminating the environmental impact of fracturing fluid and achieving better environmental benefits. Compared with hydraulic fracturing, LPG fracturing technology has longer effective fracture lengths, greatly improving single well production and ultimate recovery of hydrocarbons. Indoor numerical simulation results and actual field production data both shown that stimulation effect of LPG fracturing technology is significantly better than the same scale hydraulic fracturing.

3.2 Existing Problems of LPG Fracturing

First of all, LPG costs more than water, and the US industry has established a relatively complete system of hydraulic fracturing; manufacturers lack the motivation for alternative technology. Second, this technique is not yet mature, and its safety has yet to be approved. Since LPG is flammable and explosive gas at normal temperature and pressure, thus fracturing fluid tank, pipelines, valves and pumps all need to adapt to the characteristics of LPG during construction. Entire fracturing process is a closed loop system, at all stages of the construction process, either the tank or pipeline has a high pressure, so every process requires strict fire and explosion safety design. On January 2011, in Alberta, Canada, a fire disaster happened on a mining site using LPG fracturing technology, and three workers were burned. Gas Frac said undetected LPG leak is the culprit of the accident. Now, Gas Frac company is constantly improving technology and improve safety standards, and also hope to work in shale gas operations where environment and water resources were highly demanded.

4. Conclusions & Suggestions

（1）LPG Fracturing basically needs no water, nor to invest in wastewater treatment, greatly easing the pressure on environment and water resources and preventing formation damage, is the new direction of fracturing technology development.

（2）LPG Fracturing fluid has high backflow rate as well as long effective fracture length, and is fully compatible with reservoir fluid, greatly improving single well production of oil and gas and final recovery.

（3）Liquefied petroleum gas is of high risk and strong flammability, so safety and explosion-proof issue is critical and needs strict monitoring as well as timely and effective treatment. This will be a necessary prerequisite for the large-scale promotion of this technology.

（4）Attempt to apply this technique to reservoirs with water-shortage, water sensitivity, low pressure and low permeability.

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