Review of Completion Fracturing Technology for Shale Gas Horizontal Wells
abstract
Shale reservoirs can be used as gas source and reservoir rocks, and reservoirs have the characteristics of continuous distribution, low porosity, ultra-low permeability, and high brittleness. Natural gas in shale exists in three forms: free gas in rock pores, free gas in natural cracks, and adsorbed gas on the surface of organic minerals. These different storage mechanisms directly affect the way, speed and efficiency of shale gas development.
Global Energy Research estimates that large shale gas resources are mainly distributed in North America, Latin America, and the Asia-Pacific region. The 2012 study shows that the shale gas resources in the United States are about 415,000 x 10 ^ 520,000 x 108V, Canada is about 140000x10 ^ 170,000x10 ^ 3, and the recoverable shale gas resources in the main basins and regions of our country are about 260,000 x 108V, while the study of resource conditions in other regions is very limited.
The experience of shale gas development in the United States shows that stimulation technology, especially horizontal well fracturing technology, is crucial for shale gas development. Other important technologies include horizontal directional well drilling and reservoir description technology.
Shale gas completion
Barnett Shale is the first shale gas reservoir to be successfully developed in the United States, and it is also the most productive shale gas field in the United States. It has become a model for the development of other shale gas fields in the United States and even around the world. In the early stage of Barnett Shale development, vertical wells were used, but the production effect was not ideal. Around 2000, horizontal wells were turned to horizontal wells, and production was increased by 3-5 times. At present, almost all new shale gas wells in the United States use horizontal wells, with depths usually between 1200 and 2500 cubic meters, and a long radius of curvature < 10.~ 157300 > is used for deflection, which is convenient for later measures to transform.
The length of the horizontal section is usually between 600 and 15,000. With the advancement of horizontal well operation technology, the length of the latest horizontal section exceeds 30,000. The end of the horizontal section of most 83-towel 611 shale wells is slightly higher than the heel by 15 ^ 450 >, which is conducive to the backflow during fracturing and the flow of produced water to the lowest heel after water production.
The horizontal well completion method has undergone a transition from casing completion to naked eye completion method, see Figure 1. Early horizontal wells usually used 114.300 (bucket 1//> or 139.700 "1//> casing completion, and fracturing used drillable bridge plugs to achieve multi-stage fracturing. In 2012, foreign countries are more inclined to use naked eye horizontal well multi-stage fracturing method for completion and production, and the number of fracturing stages has gradually increased from 5 to 7 to more than 20. The following completion methods have been tried in the development of horizontal wells in Barnett Shale Gas Reservoir.
1.1 Casing ten non-cementing ten perforation ten general fracturing
Typical early completion of the Barnett Shale was the use of non-cementing casing liners in the horizontal section, no mechanical steering was used during fracturing, only in the horizontal section, a specific number of holes were shot in the casing, and a large displacement was used in the construction < 25-32 ^ 3/^; ^) to obtain the effect of limited viewership. Plugging balls and rock salts were also used in some operations to assist fluid steering between the boreholes. However, since the fluid can move freely in the annular space of the casing and eye naked, cracks may form in any part of the horizontal section.
Field microseismic monitoring also shows that the fracturing fluid sometimes diverts within the casing, but the generation and propagation of cracks are completely random, not necessarily at the perforation location of the casing. Various fracture events can occur at any point in the wellbore.
1.2 Casing ten cementing ten perforations ten limited viewership of fracturing
The first use of cementing casing in the Barnett Shale was to apply the limited viewership of fracturing to improve the distribution of fractures throughout the horizontal well section. The limited viewership of technology mainly uses high displacement and plugging ball-rock salt to implement diversion diversion. Monitoring of these wells also shows that fracture propagation is very random, and fracture direction is almost unpredictable. Early comparisons of the production effects of the above two completion methods show that although cementing cement can play a certain degree of steering between casing and formation, the production and recovery of naked eye wells are usually higher than that of casing cementing completions.
1.3 Casing ten cementing ten-stage fracturing
This is the most commonly used method in shale formations. When the well is completed, the casing in the horizontal wellbore is cemented first, and then a "bridge plug ten-perforated" multi-stage fracturing transformation is carried out, that is, the mechanical seal in the casing is achieved by cable or continuous YouTube sit-sealing bridge plug, and the annulus is mechanically turned by cementing. This process is then repeated several times to complete the multi-stage fracturing transformation on the horizontal section. When all the small sections have been treated, the composite bridge plug is drilled using continuous YouTube to reconnect the horizontal wellbore from heel to end and put it into production.
While this approach allows for segmented steering of horizontal sections, each stage is operated using continuous YouTube, perforating guns and fracking equipment, which is very expensive, inefficient and time-consuming. Studies have shown that the yield obtained by this method is also not ideal because cementing cement plugs many of the natural fractures and joints that are important for increasing gas well productivity.
1.4 naked eye ten lining pipe more than ten levels of fracturing
In 2006, a new type of naked eye well multi-stage pressure-only fertilizer completion method was applied in Texas 06 plus 0 ^ County. The process was originally proposed in 2001 to improve the time efficiency of multi-stage fracturing, reduce operating costs, and improve system reliability and repeatability. The naked eye multi-stage fracturing system uses a hydraulic set-off casing packer instead of cement cementing to isolate the various layers. The packer usually uses elastic elements to seal the naked eye wellbore, which does not require lifting or drilling or milling during production. At the same time, a sliding sleeve tool is used to form a channel in the wellbore between the packers to replace casing perforation.
These sleeve tools can be opened by hydraulic pressure or by putting (multiple > specific size of the starting ball to switch the sleeve and open the channel
These balls can be sealed in the tube between the two stages, so there is no need to use bridge plugs. The main advantage of the naked eye multi-stage fracturing system is that all fracking treatments can be completed continuously in one string without the use of bridge plugs, which greatly saves time and cost. After the production stimulation operation is completed, it can be quickly flowed back and put into production, and the ball seat can be drilled later to further increase production. The long-term production effect of Barnett Shale cementing and naked eye multi-stage fracturing completions is compared abroad.
The research shows that although in some areas where casing cementing is the mainstream, the traditional concept still insists on using casing wells, but the production effect of naked eye wells in Barnett Shale is obviously better than that of cementing. 031X611 ^ 0 ^ 06 ^ 61 et al. 2010 compared two adjacent wells with parallel horizontal wells in Barnett, and the two wells used different completion methods: the well man used naked eye multi-stage completion (naked eye ten sliding sleeves ten pitching ball fracturing); well 8 used casing cementing ten bridge plugs ten perforation completion.
The completion stages, proppant, and fracturing fluid volumes of the two wells were also relatively similar; however, during the three-year production period, the cumulative production of the wellmen was almost 2.5 times that of Well 8.
Recently, 03 ^ 611 et al. also simulated the production failure profile of level 6 naked eye and level 6 casing cementing completions, and the results show that naked eye completions can better achieve oil and gas leakage in the full horizontal section'This is because cement cementing on the horizontal section hinders the production of annulus; shale gas reserves are usually naturally fractured and joint development, and naked eye completions can greatly play the contribution of these natural fractures to production.
shale gas hydraulic fracturing
Shale is an ultra-low penetration rate reservoir, the penetration rate is mostly between 0.00. 0.01 ^ 0, so it belongs to the "nano-Darcy" penetration rate formation, and all reservoirs must be fracked before production. Usually, the cost of a single well for shale gas fracturing in the United States is more than $5 million, which is twice the cost of drilling horizontal wells. Shale gas wells use large-scale hydraulic fracturing, usually with 45~ 4501 proppant added, and the fracturing fluid is usually more than 10,000.
In 2000, large-scale use of clean water fracturing (also known as drag-reducing water fracturing) began, which was cheaper than gel hydraulic fracturing and more effective in increasing production. Many operators regard clean water fracturing as the most important revolution in the history of shale gas technology development.
2.1 Fracturing fluid
Three main liquid systems are used in shale gas fracturing in the United States: direct injection of liquid nitrogen, injection of nitrogen foam, and drag-reducing water. In addition to cost advantages, drag-reducing water can pump large amounts of water and a small amount of proppant at high discharges, and can carry the proppant into deeper fracture networks, resulting in larger fracture networks and deflation areas. It has now (2012) become standard practice in fracturing operations.
Drag-reducing water makes up the vast majority of water [99.5 ^ >, so it is also called water fracturing. Among many additives, two agents that must be added are drag-reducing agents and fungicides. The most commonly used drag-reducing agent is partially hydrolyzed acrylamide. Polyacrylamide is usually used as an external oil phase emulsion with a built-in phase inverter or an additional phase inverter [invertible emulsion or hydrated polymer], which can provide a certain viscosity and reduce friction.
In order to improve the drag reduction effect, industrial drag reducer products have high molecular weight, such as polymerizing polyacrylamide and acrylic acid to form a polymeric electrolyte, thereby improving its drag reduction rate. Since the proppant transport does not require high viscosity under the turbulent flow conditions of fracturing, it can generate conduction cracks as long as the cross-linking liquid or foam liquid at a lower cost.
2.2 Proppant
For drag-reducing water fracturing fluids, a small diameter [40/70 mesh] proppant is usually used, and a smaller particle size [100 mesh] proppant should be considered for shale formations with natural fractures. This is because the proppant in drag-reducing water and clear water has poor transmission performance, and the use of a small diameter will improve the suspension performance to a certain extent, while also obtaining a higher conductivity.
A large part of the fractures will not be supported, but due to the brittle fragmentation of shale rock, formation slippage and the embedding of proppants, an "infinite" diversion area will still be formed in the fracture system, which is the "unsupported" fracture conductivity proposed by foreign scholars.
In the early stage of drag-reducing water fracturing, some shale gas wells were fracked without sand, and the fracturing also achieved good production results. Therefore, there is still controversy in the industry as to whether proppant must be added during fracturing, but the more general understanding is that adding sand can improve the conductivity of the formation and help improve the production increase effect.
2.3 Fracturing process
Usually shale gas reservoirs are very thick, and the length of the horizontal section in shale development is gradually increasing. Hydraulic fracturing construction will be carried out in multiple stages and 7 sections on the horizontal section. Each pumping is carried out for one section of the shale gas reservoir, and each two sections are separated. In addition, the fracturing fluid in each layer is also divided into multiple slugs when pumping, aiming to form a more complex fracture network in the formation.
After the pressure test of the surface equipment, a kind of "rock acid" (usually only mouth) is pumped first to clean the near-well pollution; the second step is to inject "drag-reducing water" slug, because the drag-reducing agent is added, the water can quickly enter the formation with a large displacement; After that, a large amount of drag-reducing water and low-concentration fine sand are injected, and the sand concentration is gradually increased; when the sand addition is about to end, drag-reducing water and coarse sand are injected to keep the near-well cracks open and obtain high conductivity near the well; finally, drag-reducing water is used to wash the well and drain back to remove the sand in the equipment and wellbore.
After the well is cleaned, the downhole tool is moved to another level to start the next stage of fracturing.
In order to further improve operational efficiency and reduce operating costs, foreign countries have recently proposed the concept of "well factory" in shale gas development, that is, the well site or platform is used as a "factory" for joint operations, and drilling, cementing, perforation, multi-stage fracturing and other construction are regarded as one process on the assembly line operation. Drilling, completion and production of multiple wells are completed in the same well site. According to reports, 16 horizontal wells with a total of more than 400 stages of fracturing have been completed in one well site abroad.
This operation mode not only greatly improves the utilization rate of equipment and train sets, but also makes the fracture network generated underground more complex. Field application also shows that the production effect is 75 ^ to 130% higher than that of single well fracturing.
In fracturing, microseismic imaging and digital emulators can also be used to monitor the fracture characteristics in each shale reservoir, observe the growth trajectory of the fracture, and evaluate the fracture height, length and orientation. During construction, operators should pay particular attention to the vertical growth of cracks to ensure that hydraulic cracks do not penetrate the shale reservoir and extend to adjacent water layers.
conclusion
(1) Horizontal well drilling and completion and fracturing technology has become the mainstream technology of shale gas development. Shale gas horizontal well fracturing and completion applications are cementing perforation bridge plug fracturing and naked eye multi-stage sliding sleeve fracturing. The latter can handle more than 20 grades at a time, improving operation efficiency and better long-term production results.
(2) Drag-reducing water fracturing can increase the complexity of fractures in shale reservoirs, reduce costs, and improve production stimulation, making it the preferred liquid system for shale gas fracturing. Small-diameter proppants are usually used in magic drag-reducing water fracturing, and a large part of the fractures will not be supported. However, due to the brittleness of shale, the slippage of rock formations, and the embedding of proppants, the fracture system will still provide high "unsupported" fracture conductivity.
The development of shale gas in our country is currently in its infancy. It is necessary to integrate existing technical forces, strengthen research, and focus on solving key technologies such as downhole tools, construction fluids, and construction processes related to multi-stage fracturing technology for naked eye horizontal wells in shale gas development. To promote and realize the transformation of shale gas from "extraordinary gas" to "conventional gas".
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