Tight sandstone gas, also known as tight gas, usually means that there is no natural productivity in sandstone reservoirs with low permeability and only through large-scale fracturing or special gas production technology can natural gas with economic value be produced. This definition also applies to coalbed methane, shale gas and tight carbonate reservoir gas (Holditch, 2006). Tight sandstone gas reservoirs are mostly distributed in the center of the basin or deep in the basin structure, and are distributed continuously in a large area, so they are also called deep basin gas reservoirs, basin center gas reservoirs and continuously distributed gas reservoirs.
1. Study on tight sandstone gas
The early study on the formation of tight sandstone gas reservoirs in San Juan basin of the United States is called subtle gas reservoirs. Silver mentioned in 1950 that the basin lacked edge and bottom water and the Cretaceous strata generally contained gas. In 1970s, many researchers gave various explanations for this special type of gas reservoir, and put forward isolated (pore) gas reservoir, stratigraphic-diagenetic gas reservoir, hydrodynamic gas reservoir and water-sealed gas reservoir. 1976 Elm Worth giant deep basin gas reservoir was discovered in Alberta basin in western Canada. Until 1979, Masters put forward the concept of deep basin gas based on the analysis of Elmworth, MilkRiver and Blanco gas fields. 1986, Rose and others used the term "basin central gas" for the first time when studying Raton basin. 1979, 1980 Lao et al. and 1985 Spencer et al. studied "tight sandstone gas or tight sandstone gas". The concept of "continuous gas reservoir" was formally used in 1996 (Schmoker, 1996). After 1990s, the concepts of "deep gas" and "deep gas" appeared in China.
In 2006, the United States Federal Geological Survey put forward: deep gas, shale gas, tight gas sand, coalbed methane and shallow microbial gas sand. And natural gas hydrate or methane hydrate, collectively referred to as continuous gas.
2. Classification standard of tight sandstone gas reservoirs
(1) foreign classification standard
Due to the different resources and technical and economic conditions in different countries and regions, the definition of tight gas reservoirs has not yet formed a unified standard. 1980, according to the relevant provisions of the Natural Gas Policy Act (NGPA) of the US Congress 1978, the Federal Energy Regulatory Commission (FERC) of the United States determined that the registration standard of tight gas reservoirs was that the reservoir formation permeability was less than 0. 1× 10-3μm2. This official definition is used to determine which gas producing wells can be obtained. Elkins( 198 1) divides the reservoirs into conventional reservoirs and unconventional reservoirs with the underground permeability of 0. 1× 10-3μm2 as the boundary. Spencer (1985, 1989) defined tight gas reservoirs as gas-bearing reservoirs with in-situ permeability of natural gas less than 0. 1× 10-3μm2. Sudam (1997) proposed that tight gas refers to unconventional natural gas produced in tight sandstone reservoirs with low permeability (generally, the porosity is less than 12% and the permeability is less than 1× 10-3μm2). Stephanie Tal. (2006) It is considered that tight gas reservoirs can produce a large number of economically valuable natural gas only by hydraulic fracturing or using horizontal wells or multilateral wells. Philip H.Nelson(2009) defined tight sandstone reservoir as pore throat with a diameter of 2 ~ 0.03 microns.
(2) Domestic classification standards
There is no unified understanding of the definition and standard of tight sandstone gas reservoirs in China. Yuan (1993) thinks that tight reservoir refers to clastic reservoir with permeability less than 1× 10-3μm2. Guan Deshi et al. (1995) pointed out that tight gas reservoirs have low porosity (< 12%), relatively low permeability (0. 1× 10-3μm2), low gas saturation (< 60%) and high water saturation.
Zou Cai et al. (20 10) think that tight sandstone gas is characterized by porosity < 10%, in-situ permeability < 0. 1× 10-3 μ m2 or air permeability
(3) Classification parameters of tight sandstone gas reservoirs
Permeability is an important parameter for the division of tight sandstone gas reservoirs. In practical application, permeability adopts different definitions and reference values, such as formation permeability, air permeability, effective permeability and absolute permeability. In fact, there is a great difference between formation permeability and air permeability. Generally speaking, the increase of water saturation and the increase of overlying formation pressure will lead to a significant decline in gas permeability. When the water saturation of the rock sample is 55%, the air permeability is only 1/3 ~ 1/7 of the dry sample. When the formation pressure is 3.5 ~ 35 MPa, the formation permeability is only 1/2 ~ 1/25 of kjeldahl permeability.
It can be seen that the most important parameters of tight sandstone gas reservoirs are formation permeability, formation pressure, water saturation and porosity. But in many countries, tight gas reservoirs are defined by flow rather than permeability; Some scholars believe that the definition of tight gas reservoir should be determined by many physical and economic factors.
3. Definition and geological evaluation method of tight sandstone gas
Definition of (1) tight sandstone gas
To sum up, the definition of tight sandstone gas is: sandstone gas layer with overlying matrix permeability ≤0. 1× 10-3μm2. Generally, a single well has no natural production capacity, or the natural production capacity is lower than the lower limit of industrial gas flow, but industrial natural gas production can be obtained under certain economic conditions and technical measures. Usually, these measures include fracturing, horizontal wells and multilateral wells. The permeability of overburden matrix is determined by the crack-free core (matrix) under the action of net overburden pressure.
For the sample, the measured permeability Ki under different experimental confining pressures is divided by the conventional air permeability Ko, and normalized, and the relationship curve between (Ki/Ko) and experimental confining pressure pi is drawn. Finally, the fitting function of (Ki/Ko) and pi is used to calculate the permeability under the condition of net overlying rock pressure. On this basis, the overlying permeability is modified: firstly, the relationship curve between the overlying matrix permeability and the conventional air permeability of the sample is established; Then, the conventional air permeability of all rock samples is corrected to overburden permeability by fitting function. The relative error between the corrected overburden permeability and the measured overburden permeability should be controlled within 65438 00%. If the relative error of more than 20% samples exceeds 10%, it is necessary to re-select the fitting function or piecewise fitting.
(2) Evaluation method of tight sandstone gas
The evaluation of tight sandstone gas is divided into three levels: first, the determination of tight sandstone gas wells, the median permeability of overlying matrix of rock samples in the target interval of a single well is ≤0. 1× 10-3μm2, and the gas testing in the target interval of a single well has no natural productivity or the natural productivity is lower than the lower limit of industrial gas flow, and it reaches the lower limit of industrial gas flow after adopting technologies such as fracturing, horizontal well and multi-branch well; Secondly, the determination of tight sandstone gas layer, the median permeability of overlying matrix of all coring wells in the target interval is ≤0. 1× 10-3μm2, and the proportion of tight sandstone gas wells in all gas wells should be ≥ 80%; Finally, the geological evaluation of tight sandstone gas mainly includes resource evaluation, reservoir evaluation, reserve evaluation and productivity evaluation.
Resource evaluation: on the basis of regional geological research, comprehensive research is made by using seismic, drilling, logging, coring, analysis and testing data, so as to find out the regional structural cycle, regional sequence stratigraphic framework, distribution of sedimentary system and source rock, and determine the main gas-bearing systems, reservoir-forming combinations and trap types of regional and basin evolution; Systematic evaluation, risk analysis and queuing optimization of possible gas-bearing systems, prospective areas and key traps in the whole region; Determine the favorable area for natural gas accumulation and evaluate the resource potential.
Reservoir evaluation: on the basis of stratigraphic division, describe reservoir lithology, physical properties, heterogeneity, microscopic pore structure, clay minerals, fracture development and reservoir sensitivity. According to the reservoir physical properties, pore structure, heterogeneity and effective thickness, the tight sandstone reservoir is evaluated by comprehensively considering the reservoir shape and distribution range and combining with productivity.
Reserves evaluation: based on exploration findings, comprehensive use of various data to evaluate the main controlling factors and reserves scale of tight sandstone gas.
Productivity evaluation: according to the reserve scale and reservoir characteristics, combined with gas well production performance, determine the reasonable productivity scale.
Second, tight sandstone oil.
1. Definition of tight sandstone oil
At present, the definition and characteristics of tight sandstone oil are rarely involved in domestic and foreign documents, mainly referring to the concept of tight reservoir in some technical papers on reservoir development engineering. For example, L.Guan et al. (2006) mentioned in the article "Fast tapping method for potential of infill drilling in mature tight reservoirs" that infill drilling plays an important role in improving oil and gas recovery in tight reservoirs. Li Zhongxing et al. (2006) mentioned in the article "Key Technologies of Complex and Tight Reservoir Development" that the ultra-low permeability reservoir of Yanchang Formation in Ordos Basin has the characteristics of dense lithology, poor physical properties, small pore throat, large starting pressure gradient and easy damage, and the horizontal well perpendicular to the principal stress direction and hydraulic jet fracturing technology can initially realize the effective development of tight reservoir. BrentMiller(20 10) put forward a series of reservoir reconstruction technologies for the development of Middle Bakken shale in tight oil in the article "Opening tight oil: Selective Multi-stage Fracturing in Bakken Shale".
From the current understanding and production practice, tight sandstone oil, or tight oil, generally refers to the oil sandwiched in tight reservoirs such as fine sandstone and carbonate rocks in source rock series.
2. Research status of tight sandstone oil
(1) Research Status in Foreign Countries
Tight oil is becoming a bright spot of unconventional oil exploration in the world and another hot spot after the breakthrough of shale gas. In 2000, a major breakthrough was made in the development of Baken tight oil in Williston Basin, with a daily output of 7,000 tons. The American media called tight oil "black gold", and the discoverer Findlay won the AAPG Outstanding Explorer Award in 2006. In 2008, Bakken tight oil achieved scale development and became one of the top ten discoveries in the world. Williston basin covers an area of 34× 104km2, spanning the United States and Canada. Bakun Formation is vertically divided into 9 lithologic sections (Figure 3- 1), and the thickness of single layer is 0.5 ~ 15m. There are two sets of shale with a thickness of 5 ~12m, TOC content of14% ~10% and Ro content of 0.6% ~ 0.9%. Except for the fourth member, which is a conventional reservoir, the rest are tight reservoirs, and the second member is the main tight sandstone reservoir with a thickness of 5 ~ 10m. Pore types are mainly intergranular pores and dissolved pores, with porosity of 10% ~ 13% and permeability of (0. 1 ~ 65438+). The reservoir area is 7× 104km2, the oil layer thickness is 5 ~ 15m, the buried depth is 2590~3200m, the resource is about 566× 108t (according to USGS), and the oil quality is light, and the API is 4 1 ~ 44. In 20 10, there were 2,362 production wells in tight oil, USA, with a daily oil production of 12t and a cumulative oil production of 3 192× 104t.
Eagle Ford tight oil was discovered in 2008, and it is mainly produced in limestone mixed with shale, with a buried depth of 914 ~ 4,267m and an oil layer thickness of 30 ~ 90m. The source rock is yingtan shale and the reservoir is yingtan limestone, with porosity of 2% ~ 12% and permeability less than 0.06543.
At present, 19 tight oil basins and 4 sets of main tight oil layers have been discovered in North America. In 2009, the proven recoverable reserves in tight oil were 6.4× 108t, and the annual output was1230x104t.
(2) Domestic research status.
At present, in our country, the common concept is low permeability reservoir/reservoir, which refers to an oilfield with low porosity, small throat, poor fluid permeability and low productivity. Usually, it is necessary to rebuild the reservoir to maintain normal production.
Unconventional petroleum geology
Figure 3- 1 Bakken tight oil in Williston Basin
Exploration and development of tight reservoirs generally have the following characteristics:
(1) The reservoir has poor physical properties and low matrix permeability. Due to the low maturity of sediments, fine particles, poor sorting, high cement content and strong supergene diagenesis, the reservoir becomes very dense, with low porosity and large variation range, mostly 7% ~ 8%.
(2) According to genesis, it can be divided into primary low-permeability tight reservoirs and secondary low-permeability tight reservoirs. Generally, primary low permeability tight reservoirs are mainly affected by sedimentation, with fine grain size, high shale content and poor sorting. Most reservoirs are shallow buried and have not experienced strong diagenetic transformation. Rock has low brittleness, undeveloped fractures, high porosity and low permeability, mostly medium-high porosity and low permeability. The secondary low permeability tight reservoir is mainly the result of various diagenesis. This kind of reservoir used to be a conventional reservoir, but due to compaction and cementation, the porosity and permeability are greatly reduced, and the primary pores remain less, forming a dense layer.
(3) Small pore throat radius, high capillary pressure and high original water saturation. Generally, the water saturation is 30% ~ 40%, and some are as high as 60%. The specific gravity of crude oil is less than 0.85, the formation viscosity is less than 3 MPa·s, the clay mineral content is high, and the water sensitivity, acid sensitivity and velocity sensitivity are serious.
(4) The interaction between sand and mud in oil layer is heterogeneous. Due to the unstable sedimentary environment, the thickness of sand layer changes greatly, and the interlayer permeability changes greatly. Some sandstones have high shale content and low formation water resistivity, which brings great difficulties to the division of oil-water layers.
(5) Natural cracks are relatively developed. Because the lithology is hard and dense, there are different degrees of natural fracture systems, which are generally controlled by regional geostress and have certain directionality, which has a great influence on the oilfield development effect. Fracture is the channel of oil and gas permeability and the condition of water injection channeling, and artificial fractures are mostly in the same direction as natural fractures. Therefore, natural fractures must be paid attention to when developing low permeability sandstone oil fields.
(6) The reservoir is controlled by lithology, with poor hydrodynamic connection, insignificant edge and bottom water flooding and poor natural energy supply. Most of them rely on elastic and dissolved gas flooding, and the reservoir productivity drops rapidly, and the primary recovery ratio is only 8% ~ 12%. After water injection to maintain energy, the secondary recovery can be increased to 25% ~ 30%.
(7) Due to the low permeability and porosity, it is necessary to put into production through acidification and fracturing to obtain economic value.
(8) Due to the complex pore structure, small throat, high argillaceous content, and the existence of various water-sensitive minerals, it is vulnerable in the mining process, and the lost output can reach 30% ~ 50%. Therefore, it is very important to protect the oil layer in the whole oil recovery process.
At present, China has carried out exploration and development of low permeability tight reservoirs in Changqing, Daqing and Jilin oilfields. Changqing Oilfield has successfully developed a low permeability reservoir with a permeability of only (0.5 ~ 1.0) × 10-3μ m2 in Ordos Basin, and the oil production of a single well has reached 3 ~ 4t/d. ..