Composite Proppants Sintered from Fly Ash and their Application in Fracturing Tight Oil Reservoirs
Authors: Ren Hongda , Dong Jingfeng , Zhang Jingchun , Xu Fangzhou , Wang Bin , Liang Tianbo 
Published: August 2024

ABSTRACT:
This paper proposes a new low-density, high-strength composite proppant to address the rising cost of proppants in hydraulic fracturing operations in tight oil reservoirs. Fly ash and coal gangue are sintered with a certain concentration of bauxite and alum at 1300°C∼1500°C to form a composite proppant. Results from X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses show comparable strength between ceramic and composite proppants. Although the conductivity of new composite proppants is 15% smaller than ceramic proppants after 150 h of experiments, their strength is sufficient for effective operation under the closure pressure of 52 MPa. Field pilot tests in the Jimsar shale oil reservoir demonstrate a promising application, showcasing a two-thirds reduction in costs and a 4.5% increase in production compared to ceramic proppants. The study provides an economical and effective tool to develop tight reservoirs with depths of over 3500 m.

1. INTRODUCTION
The pursuit of recovering oil and gas from tight reservoirs requires the application of horizontal wells and hydraulic fracturing, with the pivotal role of proppants in supporting the hydraulic fractures[1]. Given the inherently low permeability of tight reservoirs (typically below 0.1 mD), effective communication within these reservoirs predominantly relies on fractures supported by proppants. The conductivity of these fractures serves as a critical factor influencing the production capacity of oil and gas[2]. Over the recent years, there has been a progressive increase in the amount of proppant injected per meter of stimulation during hydraulic fracturing operations, increasing from 1 t/m to 4∼5 t/m [3]. This increase, however, has resulted in a significant increase of the operation cost[4].
To advance the economic development of unconventional oil and gas, certain North American companies have proposed a strategy of using local natural sands. This involves the substitution of low-cost quartz sands for ceramic proppants, particularly in wells with long horizontal sections and substantial proppant injection, thereby reducing the operation cost. However, the utilization of natural sands as proppants has revealed challenges due to its material strength, leading to higher rates of breakage and subsequently compromising the conductivity of fractures, impacting the long-term production of tight oil and gas[5]. Through the proppant compressive strength testing, Zheng et al. highlighted that the crushing rate of 30/50 mesh ceramic proppants under 69 MPa closure pressure is approximately 5% lower than that of 30/50 mesh quartz sand under 35 MPa closure pressure[6]. Moreover, the conductivity of ceramic proppants is approximately 20% higher. Similar results are observed across various proppant specifications. In field experiments conducted by Mark G. Mack et al. in the Midland Basin, 20 wells are subjected to trials where 10 wells receive advanced ceramic proppants (ACP), and the other 10 receive intermediate strength proppants (ISP)[7]. Operation parameters are controlled to be as similar as possible, and an increase of production occurs within a comparable timeframe. Cumulative oil production analysis over 12 months reveals a 20% increase in oil and a 30% increase in natural gas production for wells treated with advanced ceramic proppants compared to those treated with traditional ceramic proppants. Similar growth in cumulative oil production is observed when ISP replace quartz sands. Laboratory tests demonstrate that at pressures exceeding 48 MPa, 50/60 mesh ACP exhibit better conductivity than 40/70 or 40/80 mesh ISP[7]. Furthermore, under high closure pressure, its conductivity is maintained for a longer duration.