ISSN 1062-7391, Journal of Mining Science, 2016, Vol. 52, No. 3, pp. 608–614. © Pleiades Publishing, Ltd., 2016. Original Russian Text © S.V. Serdyukov, T.V. Shilova, L.A. Rybalkin, 2016, published in Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, 2016, No. 3, pp. 192–199.

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Down-the-Hole Device for Measuring Recovery and Coal Permeability S. V. Serdyukov*, T. V. Shilova, and L. A. Rybalkin Chinakal Institute of Mining, Siberian Branch, Russian Academy of Sciences, Krasnyi pr. 54, Novosibirsk, 630091 Russia *e-mail: [email protected] Received April 1, 2016

Abstract—A down-the-hole device has been designed for gas dynamics analysis in coal. The device is manufactured based on the layout of a straddle packer with an adjustable interval. The device design is suitable for hydrofracturing and gas dynamics researches using the methods of indicator diagrams and pressure buildup and drawdown curves in package with relaxation of coal and rock mass by means of radially symmetric loading of hole walls in the hydrofracture interval. Keywords: Coal bed, hole, gas dynamics analysis, hydrofracturing, down-the-hole device. DOI: 10.1134/S1062739116030886

Gas dynamics analysis in mines is intended to show pressure and content of gas in a coal bed, permeability of rocks and prompt detection of bleeders. These data are required to forecast gas inflow in mines and to design efficient pre-mine drainage. In the known methods and means for the gas dynamics analysis in mines, yield, temperature and pressure of gas is measured at the well mouth, including drilling. The mouth measurements allow integral value of the measured characteristic per a well, which complicates mapping of gas dynamic and flow characteristics of a coal bed. This issue becomes even more critical in deep directional drilling of holes more than 1000 m long [1]. Another obstacle is uneven gas flow to the hole and to a production face area. Mainly, gas liberates under hole drilling. Gas measurements taken during drilling and in core testing show coalbed gas content and not the gas dynamic properties of coal and rock mass subjected to high stresses and with small and extra compressed exposed surface at the drill hole contour. This is largely different from the conditions in the production face area and cannot be used for gas inflow forecasting there. More relevant data are obtained from the gas dynamics analysis performed in the holes in the area of controlled coal destressing [2]. Such methods allow coalbed gas yield versus compression (Fig. 1) but their application is limited to rocks surrounding mine openings. This article describes some theoretical and applied approaches to gas dynamics researchers in combination with destressing far from the long hole mouths.

Fig. 1. Gas yield g of a short hole versus compression stress σ in the borehole slotter zone in coal bed [2].

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Destressing in the gas measurement interval is implemented through the local hydraulic fracturing and opening of the created fracture to a pre-set depth by the adjustable radial-symmetrical loading applied to the fracture surfaces. This enables measuring coalbed has yield from the fracture surface area that is stress-free. This surface is exposed after sealing the fracking interval, which offers options of studying gas yield dynamics off the influence zone of drilling. Efficiency of the approach proposed to analyze coalbed gas dynamics will be determined based on field trials. This article deals only with implementation of the approach. It is difficult and costly to perform observations at a distance from mouths of uncased horizontal and vertical holes. Aimed at cost-saving, the newly designed device is equipped with a built-in robotic system to advance the device along the hole and with solid flexible lines for connection with the pumping and measurement equipment installed in an underground excavation. The same approach was used in the design of a downhole precision dilatometer [3]. The discussed device is designed to seal short intervals in an uncased holes when pressure fluid is fed in this interval (compressed nitrogen, water and oil emulsion), or when pressure fluid is pumped out with measurement of the fluid pressure, temperature and vole flow rate. The device implements such operations and measurements as: —hydraulic fracturing of coal bed with the measurement of the shut-in pressure Ps and re-opening pressure Pr; —increment or drop in the pressure in the sealed interval of the hole and, then, measurement of the pressure drawdown or buildup P(t); —determination of an indicator diagram—pressure in the sealed interval versus reservoir fluid flow rate Q from this interval. In this manner, the device aids in gas dynamics analysis in a local zone of a coal bed, both in the stationary (indicator diagram) and dynamic (pressure drawdown curve, pressure buildup curve) modes. With various methods used, the device improves reliability of measurements of permeability and pore characteristics, including formation pressure, gas permeability and gas flow rate and predicted yield of gas drainage holes in a coal bed. The feature of the device is the option of the gas dynamics analysis far from the mouths of long holes with walls subjected to radial-symmetric loading. The information about the state of a coal bed in advance of extraction assists in adjustment of mining schemes in coal with high gas content toward the reduction in hazardous gas-dynamic events. The layout and hydraulics of the proposed device ate illustrated in Figs. 2 and 3, respectively. The device is manufactured as double packers with an adjustable gap 3 between packer shells 1 and 4 reinforced with a cord. Adjustment of the gap 3 is made using the hydraulic cylinder 7. The unmovable end of the shorter shell 4 is fixed on the cylinder housing. The longer shell 1 is fixedly attached to the polished shaft 5 of the cylinder. The other, movable ends of the shells are pressuretight slidable along the shaft.

Fig. 2. The down-the-hole devices for gas dynamics analysis in coal and rocks: 1—double-length packer shell fixedly attached to the hydraulic cylinder shaft; 2—drainage layer; 3—packer-to-packer gap; 4—packer shell fixedly attached to the cylinder housing; 5—cylinder shaft; 6—cylinder piston; 7—hydraulic cylinder; 8—gas strut; X—fixed end of the packer shell; •—packer shell end pressure-tight movable along shaft 5; L1–L3—connection lines for the device and the equipment in underground excavations. JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016

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Fig. 3. Hydraulic circuit of gas dynamics analysis equipment: A1—down-the-hole device; A2—onboard station; P1, P2—packers; C—travel cylinder; D1–D3—distributors; I—isolator; PRV—pressure relieving valve; T—throttle; S1—high-pressure sensor; S2—low-pressure sensor (vacuum gauge); TS—temperature sensor; FR—gas flow rate meter.

The specifications of the device are given below: Length Diameter Weight Packers Length of packers High-pressure connection hoses (HPH) Flow area of HPH Hydrofracturing pressure Packer-induced pressure on hole walls Hole diameter Hole length Hole orientation Pressure meter range Measurement frequency Pressure measurement accuracy (per range) Formation fluid temperature Temperature measurement accuracy Formation fluid flow rate

Continuous operation duration Advance rate in hole

2870 mm 60 mm 12 kg 2 500 and 1000 mm 3 6 mm To 18 MPa To 26 MPa 76–105 mm To 1000 m Unlimited I: 0–0.1; II: 0.1–18 MPa 0–1000 Hz I: 0.001; II: 0.02 MPa – 10 ÷ 80 °С 0.001 °С Measured by removable flow rate meter built-in in the board station Unlimited To 100 m/h

The device has three operating modes: advance along the hole, hydraulic fracturing, fracture widening by the packer shells and gas dynamics analysis. To advance the device along a drill hole, the power fluid (oil and water emulsion, or compressed nitrogen) is fed under pressure, through the distributor D1 and the high-pressure flexible hose (HPH) L2, to the packer shell B 2 and cylinder C (see Fig. 3). The pressurized fluid inflates the shell B2 and makes it stick together with rock. This intergagement serves an anchorage for the cylinder housing. JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016

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The power fluid pushes the piston 6 (Fig. 2) and the shaft 5 of the cylinder depth-ward the drill hole and drags the packer shell B1 and HPH L1–L3. At the next step, the power fluid, via D2 and HPH L1 (Fig. 3) is fed in the packer shell B1, inflates it and makes it stick together with the drill hole walls. The pressure in the packer shell B2 is relieved using the line L2–distributor D1–discharge system. The shell B2 loses adherence with rocks and is displaced relative to the immobile shaft depth-ward the hole together with the cylinder housing by the gas strut 8. These operations are repeated until the device comes to the preset interval in the hole. The power fluid pressure to advance the device is selected based in the orientation and length of a hole. For holes 800 m long and oriented vertically upward, the pressure of 6–8 MPa is sufficient. The maximum advance rate of the device in holes reaches 100 m/h. For fracking, the power fluid is fed under the pressure of 2–4 MPa first in the packer shell B2 and, then, in the both shells B1 and B2. This sequence of the fluid feed ensures the maximum size of the packer gap. Later on, the power fluid is pumped in the both shells and concurrently, using the pressure relieving valve and HPH L3, in the fracturing interval of the hole until creation of a fracture in surrounding rocks (Fig. 4a). PRV maintains pressure difference of 4–6 MPa between the packers and the packer gap. This is sufficient to seal the fracturing interval. The sensor S1 measures pressure in PRV L3. Based on the senor data, the shut-in Ps and reopening Pr pressures are determined and then used to evaluate the required destressing and drainage area in coal bed. To carry out the gas dynamics analysis, the device is advanced along the drill hole so that the packer shell B1 part covered with a drainage layer occurs in the drill hole interval with the created fracture (Fig. 4b). After that, the power fluid under the pressure of 2–4 MPa is fed in the packer shell B1 and, then, in the both packer shells B1 and B2. In this case, this sequence of the fluid feed ensures the minimized size of the packer gap. The gas dynamic analysis by the device includes the indicator diagrams, pressure drawdown curves and pressure buildup curves. In the method of the indicator diagrams, the packer gap, using HPH L3, distributor D3 and isolator I, is connected to the mine drainage system. Gas flow rate is controlled using the throttle T. The temperature, pressure and flow rate of gas are measured by the related sensors (Fig. 3).

Fig. 4. Operation of the device in the modes of (a) hydraulic fracturing and (b) gas dynamics analysis: 1—created fracture; 2—zone of the fracture widening by the packer shell; 3—gas inflow in the packer gap. JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016

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Fig. 5. Fracture growth length L per units of the drill hole radius R under radial-symmetrical loading of the hole well by the packer shell with the pressure P [5].

In the pressure buildup curve method, the isolator is first completely opened and, then, after gas yield stabilization, is completely closed, and the pressure buildup is measured in the packer gap. In the pressure drawdown curve method, the isolator is closed, and compressed nitrogen is fed under pressure in the packer gap by the distributor D3, the feed time is measured. After the nitrogen feed is stopped, the pressure drop in the packer gap is measured by the sensors S1 and S2 as against time. The gas dynamics analysis using any of the above described methods is possible at any values of the pressure P in the packer shell B1 within the ranges P < Pr and P > Pr when the created fracture is closed or partly opened, respectively. The stress at the hole contour at P < Pr is calculated using the measured values of Ps and Pr [4]. Within the range P > Pr , the fracture growth L is evaluated using the plots in Fig. 5 [5]. The parameter  in Fig. 5 is a ratio of the minimum and maximum compressions in rock in the plane orthogonal to the drill hole axis, given below in terms of the measured values of Ps and Pr : Ps . α= 3Ps − Pr The data of the gas dynamics analysis carried out with the developed devices and using the method of indicator diagrams can be processed with the procedure below: —the measured data are used to plot ( P0 − P) / Qg versus the outlet gas flow rate Qg , where P—

packer gap pressure; P0—formation pressure determined based on stabilization of pressure in the sealed packed gap when Qg = 0 ; —the resultant plot is approximated by a straight line: P02 − P 2 = a + bQg , Qg where a, b—indexes to be found; —the maximum expected gas yield when the hole is connected to the mine drainage system is calculated as: a 2 + 4b( P02 − PV2 ) − a max Qg = , 2b where PV—mine drainage system pressure; —the formation pressure P0 is found using the formula: P0 =

1 N

N

 n =1

Pn2 + aQg n + bQg2n . JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016

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In case that the resultant formation pressure greatly differs from the value used to plot the indicator diagram, the average pressure calculated from (1) is taken to re-process the data in accordance with the presented algorithm. The pressure buildup curve data are processed using the simplified procedure [6]: —the obtained data are taken to determine gas flow rate before the research and to plot the relationship between the squared pressure value in the packer gap and the common log time t counted as seconds from the beginning of pressure buildup; —the relationship is approximated by the relation: P 2 = γ + β log t ,

where γ and β —coefficients of approximation; — β is used to calculate coalbed permeability: 0.023 qT0 Z 0 PV , π β Ts where q—gas flow rate before the test using the pressure buildup curve method; T0 —coalbed gas

εg =

temperature; Ts = 293 K; Z 0 —methane compressibility index at the values of the formation pressure and temperature; —the known value of the dynamic viscosity of coalbed, μ , is used to calculate coalbed permeability per unit length of the packer gap, h: kg =

εgμ h

.

The data output are the transmissibility and permeability of coalbed. For processing of measurement results, it is possible to find better, advanced procedures, which are beyond the present discussion. It is noteworthy that coalbed methane can be free and occluded. During coal mining, free methane quickly comes in the zone of low pressure, while occluded methane slowly diffuses in pores of coal. As a consequence, there are two branches in the pressure buildup curves, and they are interpreted separately. Diffusion is a slow process; therefore, testing may take hours and days, which is not always acceptable in borehole section surveys. An alternative approach is a quick test with gas yield evaluated relative to free methane only. Overall gas content of coalbed can be determined in this case from calculated ratios of free and occluded methane, e.g. using a procedure described in [7]. CONCLUSIONS

The designed device provides gas dynamic analysis of coalbed in stationary and nostationary modes using the methods of indicator diagrams and pressure buildup and drawdown curves. Feasibility of different methods of analysis improves accuracy of determination of porosity and permeability characteristics of coalbed, including formation pressure, transmissibility and estimated yield of drainage wells. A feature of the device is capability to perform measurements far from drill hole mouth. The device is fitted with an inbuilt system for advance in long uncased holes of any orientation and is capable to operate efficiently at a distance to 1000 m from roadways. JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016

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The device performs hydraulic fracturing and gas dynamic analysis together with widening of the created fracture by applying radially symmetric load on the fracture surfaces. The associated local destressing of coal favors higher reliable analysis of coalbed gas content and allows studying gas yield versus stress-free area of gas drainage surface. ACKNOWLEDGMENTS

The work was supported by the Ministry of Education and Science of the Russian Federation, project no. RFMEF160414X0096.

REFERENCES 1. Uskov, A.V. and Voitov, M.D., Directional Hole Drilling for Premine Coal Drainage, Vestn. KuzGTU, 2010, no. 3, pp. 33–34. 2. Bol’shinsky, M.I., Lysikov, A.B., and Kaplyukhin, A.A., Gazodinamicheskie yavleniya v shakhtakh (GasDynamic Events in Mines), Sebastopol: Veber, 2003. 3. Serdyukov, S.V., Degtyareva, N.V., Patutin, A.V., and Rybalkin, L.A., Precision Dilatometer with Built-In System of Advance along the Borehole, J. Min. Sci., 2015, vol. 51, no. 4, pp. 860–864. 4. Shkuratnik, V.L. and Nikolenko, P.V., Metody opredeleniya napryazhenno-deformirovannogo sostoyaniya massiva gornykh porod: nauch.-obrazovat. kurs (Methods to Determine Stress State of Rocks: Education and Research Course), Moscow: MGGU, 2012. 5. Martynyuk, P.A., Pavlov, V.A., and Serdyukov, S.V., Assessment of Stress State in Rocks by Deformation Characteristic of Borehole Zone with Hydrofracture, J. Min. Sci., 2011, vol. 47, no. 3, pp. 290–296. 6. Serdyukov, S.V., Patutin, A.V., Shilova, T.V. et al., Metodika kompleksnykh geofizicheskikh skvazhinnykh issledovanii gazonosnykh ugol’nykh plastov: otchet o NIR (Procedure for Integrated Geophysical Borehole Surveys in Gaseous Coal Beds: R&D Report), Novosibirsk: 2013. 7. Fel’dman, E.P., Vasilenko, T.A., and Kalugina, N.A., Physical Kinetics of Coal–Methane System: Mass Transfer, Pre-Outburst Events, J. Min. Sci., 2014, vol. 50, no. 3, pp. 448–464.

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