Borehole Flow Research Articles

Sampling in Long-Screened Wells: Issues, Misconceptions, and Solutions.

The issues associated with long-screened wells (LSWs) (and open boreholes) at contaminated sites are well documented in the groundwater literature but are still not fully appreciated in practice. As established in seminal and review papers going back over three decades, the interpretation of sampling results from LSWs is challenging in the presence of vertical hydraulic gradients and borehole flow; furthermore, LSWs allow for vertical redistribution of contamination between aquifer layers. Acknowledgment of these issues has led to the development of new technologies and well designs to enable discrete-zone monitoring (DZM), yet LSWs remain common for many reasons, for example, as multipurpose wells, for geophysical logging, and (or) as legacy installations. Despite the literature on LSWs and despite the adoption of DZM at many sites, the use of LSWs persists and the challenges of interpreting sampling results from LSWs remain. In this issue paper, we provide a conceptual overview of the problems posed by LSWs and review existing literature and past work to improve the interpretation of sampling in LSWs. We draw on experience from previous studies at the Hanford Site in eastern WA, USA, and use synthetic examples to illustrate key concepts and challenges for interpretation. A recently published analytical modeling framework is used to develop illustrative synthetic examples and demonstrate a workflow for building scientific intuition to understand issues around interpreting samples from LSWs, which is critical to effective characterization and groundwater remediation at sites with LSWs.

Groundwater Potential Assessment in the Rural Loulouni Municipality (Southern Mali): Decision Tree Approach

Groundwater Potential Assessment in the Rural Loulouni Municipality (Southern Mali): Decision Tree Approach

A Monte‐Carlo Chemical Budget Approach to Assess Ambient Groundwater Flow in Bedrock Open Boreholes

AbstractIn low‐permeability rocks, ambient groundwater flow in open boreholes may go undetected using conventional borehole‐flowmeter tools and alternative approaches may be needed to identify flow. Understanding ambient flow in open boreholes is important for tracking of cross contamination in groundwater. Chlorinated volatile organic compound (CVOC) concentrations from three open boreholes set in a crystalline‐rock aquifer (two of three open boreholes) and a siltstone aquifer (one of three open boreholes) were examined using a new approach and associated software program called the AFCE (Aqueous‐Flow‐Concentration‐Estimator). The program allows comparison of coupled chemical datasets through a Monte‐Carlo simulation and a chemical‐budget approach to assess ambient groundwater flow in open boreholes. The coupled datasets required for the comparison include aqueous CVOC concentrations from groundwater samples from (1) discrete fractures, such as those measured from temporary deployment of straddle‐borehole packer assemblies; and (2) the concentration of the open borehole (wellbore) water, as measured by a vertical profile of passive samplers from within the same open borehole. Because results from the passive samplers represent a composite mixture of the results from the discrete samples under ambient groundwater‐flow conditions, potentially at unknown proportions, the comparison between coupled datasets affords the ability to discern likely water contributions of CVOC from discrete fractures (or fracture zones), and which fractures may be dominating the water chemistry of the open borehole.

Fracturing and Groundwater Productivity Linkages in the Koumfab Watershed in Northern Togo

The Koumfab watershed, located in the Dapaong region of northern Togo, overlies the sandstone-pelite formations of the lower megasequence of the Volta Basin and the granito-migmatitic bedrock of the south-eastern edge of the West African Craton. All of these Precambrian formations have a dense network of lineaments or fractures, giving them the characteristics of discontinuous and highly exploited aquifers. The lineament map of the Koumfab watershed, obtained from aerial photographs and the digital terrain model, identifies three main families of fractures running N-S to NE-SW (N0° to N40°), NE-SW to ESE-WNW (N60° to N110°) and ESE-WNW to SE-NW (N120° to N180°). Analysis of the correlation between lineaments and borehole flow rates has enabled us to identify areas with high storage potential. This potential is mainly controlled by lithological and lineament parameters. Boreholes drilled along kilometre-long lineaments underlain by crystalline and sandstone formations are more productive, with generally high flow rates (Q > 5m3/h). The basement formations found throughout the Koumfab catchment are major reservoirs with hydrogeological potential that are already in great demand.

Triassic source rocks of the Eastern Ciscaucasia

Background. Triassic deposits of the Eastern Ciscaucasia belong to the upper part of the transient zone, which is considered promising for hydrocarbon exploration. The presence of numerous deposits, as well as non-commercial hydrocarbon flows in boreholes, indicate the existence of petroleum systems in this part of the section. However, issues related to the sources of hydrocarbons in the Triassic deposits still remain unresolved.Aim. To identify and study the possible source rocks for the Triassic deposits of the Eastern Ciscaucasia, as well as to assess their contribution to the hydrocarbon potential of the upper part of the transient zone.Materials and methods. Research materials involved all available geological, geophysical and geochemical data published on the section under study, as well as reports from the Russian Federation geological repository. The collected data were analyzed as part of the conducted basin analysis. Numerical basin modeling and paleogeographic reconstructions were performed.Results. The paleogeographic conditions of the Eastern Ciscaucasia and the adjacent areas of the Caspian Sea at the Triassic period were studied. The results of basin analysis were used to construct the distribution maps of Neftekumskaya, Kultaisko-Demyanovskaya and Kizlyarskaya source rocks and to assess their charging ability for Lower and Middle Triassic reservoirs.Conclusion. The conducted numerical modelling revealed several pods of still active source rocks in the Neftekumskaya, Kultaisko-Demyanovskaya and Kizlyarskaya sections. Lower Triassic reservoirs charged from both the Neftekumskaya and Kultaisko-Demyanovskaya source rocks, unlike Middle Triassic reservoirs that charged from the Kizlyarskaya source rocks. Thus, independent petroleum systems separated by the Anisian seal at the two stratigraphic levels in the Triassic part of the transitional complex of Eastern Ciscaucasia were identified.

Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods

Abstract In Ghana, crystalline rock aquifers with secondary hydraulic features and preferential flow pathways serve as very important aquifers. Protecting and managing these geological porous media require identifying preferential flow pathways and hydraulic characteristics. In this study, preferential flow pathways and hydraulic parameters were characterised for two boreholes (Hilly and Valley) located in Tarkwa, Ghana via the integrated use of borehole dilution testing (BDT), slug testing, and geological mapping. The geological fieldwork mapped a relatively sparsely fractured Sandstone Unit overlying a heavily foliated and fissile Phyllite Unit. Geology influenced groundwater flow in boreholes. The BDT confirmed the geology by showing a stratified flow system with preferential flow pathways. The Hilly/recharge area borehole shows relatively low downward flow, whereas the Valley/transition area borehole is dominated by the concentrated diffuse horizontal flow. These flow patterns are in agreement with the borehole locations and their dominant geologies. The estimated hydraulic conductivity, transmissivity, and storage coefficient ranged between 5.1 × 10−4–7.7 × 10−2 m/d, 4.0 × 10−2–3.2 m2/d, and 10−7–10−5, respectively. The cheap integrated hydrogeophysical methods used in this study are applicable for characterising, protecting, and managing other crystalline aquifers in the West African sub-region and other tropical terrains.

The Calculation of Heat Flow in New Boreholes in Menderes Massif: İncirliova, Aydın

Bu çalışma kapsamında Ege Bölgesi, Aydın ili sınırları içerisinde yer alan İncirliova bölgesinde yeni açılan iki kuyuda (Efeler 1 ve Efeler 2), kuyu içi termal bilgilerine dayanılarak kabuğun üst derinlikleri için ısı akısı hesabı yapılmıştır. Söz konusu bu kuyulara ait elde edilen 53,9±0,08 oC/km ve 40±0,12 oC/km jeotermal gradyan ile kuyu litolojisi incelenerek ve efektik yöntem kullanılarak hesaplanan ısıl iletkenlik değerleri ve ısı akısı değerleri ilk defa bu çalışma ile ortaya konmuştur. Efeler 1 kuyusunda elde edilen jeotermal gradyan değerleri 50 – 76°C arasında değişmektedir. Efeler 1 kuyusu düşey litolojik bilgisine dayanılarak sırasıyla 50,82 oC/km, 52,72 oC/km, 63,04 oC/km ve 75,94 oC/km değerlerine sahip olan dört ayrı tabaka olarak sınıflandırılmıştır. 2,75 W/moC ısı iletkenliği ve 53,9±0,08 oC/km ortalama jeotermal gradyan kullanılarak Efeler 1 kuyusu için efektif yöntem ile ısı akısı 148,23 mW/m2 olarak hesaplanmıştır. Efeler 2 kuyusu için 2,75 W/moC ısı iletkenliği ve 40±0,12 oC/km ortalama jeotermal gradyan kullanılarak ısı akısı 110.0 mW/m2 olarak hesaplanmıştır. Bu sonuçlar bölgede yapılan diğer çalışmalarla uyumludur ve çalışma alanı olan Aydın bölgesinin yüksek ısı akısı bölgesi olarak nitelendirilmesini desteklemektedir. Ayrıca, Curie Nokta Derinliğinden elde edilen 49,57 oC/km jeotermal gradyan ile kuyu litolojisine dayanılarak efektif yöntemle hesaplanan 2,75 W/m oC değerine sahip ısıl iletkenlik değeri kullanılmıştır ve bölge için ısı akısı 136,3 mW/m2 olarak hesaplanmıştır. Curie Nokta Derinliği ile hesaplanan ısı akısı yeni açılan kuyulardan hesaplanan ısı akısı değerleri ile karşılaştırıldığında; Curie Nokta Derinliği ile elde edilen ısı akısının kuyu (Efeler 1) ısı akısı değerinden biraz daha düşük olduğu görülmüştür. Bu durum kabuğun daha büyük bir bölümü için hesaplanan jeotermal gradyandan kaynaklanmaktadır.

Interpreting Concentrations Sampled in Long-Screened Wells with Borehole Flow: An Inverse Modeling Approach.

New approaches are needed to assess contaminant mass based on samples from long-screened wells and open boreholes (LSW&OB). The interpretation of concentration samples collected in LSW&OB is complicated in the presence of vertical flow within the well. In the absence of pumping (i.e., ambient conditions), the well provides a conduit for flow to occur between aquifer layers or fractures as a result of head differences. Under pumping conditions, vertical borehole flow may vary with depth depending on far-field heads and hydraulic conductivity; furthermore, if pumping fails to overcome ambient gradients, outflow from the well to the aquifer may occur. Concentration samples thus represent flow-weighted averages of formation concentrations, but the averaging process is commonly unknown or difficult to identify. Recognition of the importance of borehole flow has motivated the use of multi-level wells, packers, and well liners; however, LSW&OB remain common for numerous reasons, including cost, multi-purpose design requirements (e.g., pump-and-treat, water supply), logging, and installation of instrumentation. Here, we present a simple analytical model for flow and transport within a well and interaction with the surrounding aquifer. We formulate an inverse problem to estimate formation concentration based on sampled concentrations and data from flowmeter logs. The approach is demonstrated using synthetic examples. Our results (1) underscore the importance of interpreting sampled concentrations within the context of hydraulic conditions and aquifer/well exchange; (2) demonstrate the value of flowmeter measurements for this purpose; and (3) point to the potential of the new inverse approach to better interpret results from samples collected in LSW&OB.

Improvements in Test Procedure and Data Reduction for the Borehole Erosion Test

Abstract As the use of the borehole erosion test (BET) is increasing worldwide, lessons are being learned and procedures are being updated and refined. This article presents new developments including a more detailed field-testing procedure and recommendations, more advanced numerical simulations with the mesh morphing technique, an expanded data reduction process that now includes the computation of the borehole wall shear stress, more comparisons with the erosion function apparatus (EFA) laboratory test, the difference between using water and drilling mud, and advantages and limitations of the BET. The numerical simulations give the velocity and shear stress fields on and near the borehole wall associated with the water flow. They show how the bottom BET is like an in situ jet test while the lateral BET is a vertical hole erosion test. They also show that the bottom two diameters of the borehole flow do not develop a steady shear stress on the wall and should not be used. The data reduction process is expanded to include the profile of the shear stress on the borehole wall using simplifying assumptions. The comparison between the BET and the EFA indicates that the BET gives a soil erosion resistance higher than the EFA does; this may be attributed to sample disturbance for the EFA and testing under favorable in situ stresses for the BET. Two parallel BETs, one with water and one with drilling mud, shows 10 % more erosion with water than with the drilling mud. The main advantage of the BET is soil erodibility profiling through the borehole radius versus depth profile. The BET is not as useful to determine the erosion functions of the soil stratigraphy but more useful in identifying which layers are more susceptible to erosion. The BET profile helps to identify the location of samples for erosion laboratory testing.

CHARACTERIZATION OF AQUIFER STRUCTURES IN THE NORTHEAST BASEMENT ROCK OF BURKINA FASO

This characterization study of the aquifer horizons is based on the geostatistical analysis of the drilling data in order to map the intrinsic parameters of the aquifers, namely the size of the alterations and the size of the fissured fringe. The STRM data were also used to map the topographic levels. Cross-analysis of all these parameters with borehole flow rates allowed us to define a context pattern that is favorable to groundwater deposits, by identifying the influence of the size of these parameters on productivity. The application of the geophysical method (electric drag and geoelectric tomography) has made it possible to obtain a 2D vertical section of the subsoil structure based on the distribution of electrical resistivity. The alteration profile includes the alterite reservoir, the fissure reservoir located above the solid rock sometimes affected by fractures (fault). The size of these different horizons varies from one point to another, thus defining contexts that are more or less favorable for the existence of groundwater deposits. The alteration thickness (AT) mapping defined two map units: alteration thicknesses of 15m or less and those greater than 15m. Erosion would have played a large role in this distribution as there is a clear link between alteration thickness and topography. The alteration thickness (AT) unit of less than or equal to 15 m has great surface water potential. It is the area where the topography is fairly low, and the fractured edges are generally of considerable thickness. They frequently coincide with the large shallow areas drained by the major waterways.

A multiphysics field-scale investigation of gas pre-drainage in sorptive sediments

Gas pre-drainage through underground-to-inseam (UIS) horizontal boreholes is one of the common practices for the management of environmental and safety concerns in underground coal mining. In the design of UIS boreholes, local experience is invaluable; however, an experienced-based design is not necessarily optimal from both economic and technical perspective. In this study, we present an informed, data-driven design and optimization methodology that employs a multiphysics coal seam gas flow model for simulating gas pre-drainage on different scenarios of UIS borehole pattern. The implementation of the methodology is presented through a case study from an Australian underground coal mine. The methodology uses an in-house three-dimensional finite element (FEM) model (referred to as NETCoal) that has been developed based on the non-equilibrium thermodynamics and continuum mechanics. NETCoal considers the interacting processes of matrix gas diffusion, fracture gas flow and rock stress evolution in a coupled manner. This allows the permeability tensor of each grid element to be updated in response to evolving stresses on fractures induced by matrix shrinkage due to gas desorption (sorptive-mechanical effect) and gas pressure depletion (hydro-mechanical effect). The proposed methodology for the UIS borehole design requires a three-dimensional geological/structural grid model to be constructed for the area under consideration using available field data. Material and flow properties of coal (e.g., Isotherm, flow, mechanical properties etc.) are then calculated particularly using downhole geophysical logs (DGL) and physical equations/empirical correlations at borehole locations and distributed onto other grid cells using the kriging method to build a property model. NETCoal imports these geological and property models in a corner point gridding format and generates its own finite element mesh that suits the UIS borehole pattern under consideration. A Warren-Root type shape factor is used for the calibration of the numerical results of gas content with actual borehole flow data. Once calibrated, the model simulates gas drainage for different scenarios to determine an optimal borehole pattern which is the one satisfying the threshold limit value (TLV) and lead time constraints while requiring least drilling length. In addition to introducing an innovative practical methodology for the design of UIS pre-drainage boreholes, this study provides insights into the effect of coal seam and geometry parameters on gas pre-drainage through a sensitivity analysis.

Special Borehole Flow Regulator for Gas-lift Wells

Offshore gas conditioning and its controlled injection to gas-lift wells is difficult and process-critical. On the other hand, maintaining the bottomhole pressure in gas-lift wells is an important technological issue. The article is devoted to the development of a special borehole installation - flow regulator that controls the supply (injection) of gas to the gas-lift well and that ensures the bottomhole pressure in the well is maintained at a provided value. Keywords: Gas lift well; Pressure gradient; Flow regulator; Liquid-gas mixture; Valve

Quantification of time-varying groundwater flow in boreholes in fractured crystalline rock using long-term distributed temperature sensing

Quantification of time-varying groundwater flow in boreholes in fractured crystalline rock using long-term distributed temperature sensing

Quantification of time-varying groundwater flow in boreholes in fractured crystalline rock using long-term distributed temperature sensing

Quantification of time-varying groundwater flow in boreholes in fractured crystalline rock using long-term distributed temperature sensing

NITRITE CONTENT IN GROUNDWATER RESOURCES IN NIGER: CASE OF DISCONTINUOUS AQUIFERS OF THE DAMAGARAMMOUNIO BASEMENT

The base of Damagaram-Mounio outcrops in the eastern part of the Zinder region. In this zone, the aquifers are discontinuous and the groundwater resources are located in fractured, fissured and / or altered horizons of granite (Older granites) and metamorphic formations of the Pan-African and young cracked granites (Younger granites) . These aquifers are mainly captured by boreholes whose failure rates, very high, can exceed 50%. In addition, the positive borehole flows are less than 2 m3 / h at more than 85% of them. Thus, the problem of water resources arises acutely. In this densely populated area, this lack of water to feed the population is aggravated by high levels of nitrites. In fact, these levels greatly exceed the standards of drinking water for human consumption hence the present study, the main objective of which is to characterize the nitrite contents in the groundwater resources of the area. The methodological approach based on the inventory of hydraulic structures capturing the boreholes in the base and the analysis of the physico-chemical parameters of the water from several boreholes, made it possible to establish that 32% of the water from the sampled boreholes has nitrite contents. , exceeding the WHO standard set at 0.1 mg / L, therefore, these waters are unfit for human consumption. This study made it possible to highlight waters with high nitrite contents which are characterized by values ​​greater than 1 mg / L. This concerns borehole water from localities such as: Bourbourwa, Midik and that of ZengonSoumagaila. The high nitrite contents observed in the groundwater in the area may be linked to an organic pollution phenomenon. Furthermore, no significant correlation was observed between the nitrite contents and the values ​​of the electrical conductivity of the groundwater in the area.

Preprocessing approaches in machine-learning-based groundwater potential mapping: an application to the Koulikoro and Bamako regions, Mali

Abstract. Groundwater is crucial for domestic supplies in the Sahel, where the strategic importance of aquifers will increase in the coming years due to climate change. Groundwater potential mapping is a valuable tool to underpin water management in the region and, hence, to improve drinking water access. This paper presents a machine learning method to map groundwater potential. This is illustrated through its application in two administrative regions of Mali. A set of explanatory variables for the presence of groundwater is developed first. Scaling methods (standardization, normalization, maximum absolute value and max–min scaling) are used to avoid the pitfalls associated with reclassification. Noisy, collinear and counterproductive variables are identified and excluded from the input dataset. A total of 20 machine learning classifiers are then trained and tested on a large borehole database (n=3345) in order to find meaningful correlations between the presence or absence of groundwater and the explanatory variables. Maximum absolute value and standardization proved the most efficient scaling techniques, while tree-based algorithms (accuracy >0.85) consistently outperformed other classifiers. The borehole flow rate data were then used to calibrate the results beyond standard machine learning metrics, thereby adding robustness to the predictions. The southern part of the study area presents the better groundwater prospect, which is consistent with the geological and climatic setting. Outcomes lead to three major conclusions: (1) picking the best performers out of a large number of machine learning classifiers is recommended as a good methodological practice, (2) standard machine learning metrics should be complemented with additional hydrogeological indicators whenever possible and (3) variable scaling contributes to minimize expert bias.

Physical-mathematical model applied in the study and simulation of dissolution of sodium chloride particles in brines in upward flow in tubes

Drilling trough salt layers has been a challenge for the oil drilling industry. In this way, water-based muds are a friendly environment, although they tend to dissolve generated salt particles along with the wall of the well. Particularly, the particle dissolution may cause changes in the fluid rheology and physicochemical properties. Other problems related to this are the enlarging of the borehole, the excessive torque, the accumulation of salt gravels in the bottom hole, the well controlling issues, and some other operational situations. Thus, the controlling and prediction of the salt dissolution process in drilling fluids are of great interest. The main objective of this work was to study the dissolution of halite (NaCl) particles in brines at different concentrations on upward flow in tubes that simulate the borehole flow. An experimental apparatus was built, capable of reproducing some drilling conditions as the upward fluid flow where the data of NaCl concentration as function of position and time were obtained. A mass conservation mathematical model based on the continuum mechanic was proposed in order to simulate the process. The model was composed of two differential partial equations generated by the conservation law for the mass of salt in two phases: a fluid phase of salt dissolved in water and a solid phase of salt particles. The dissolution data were used to estimate the global mass transfer coefficient in different operational conditions and to validate the mathematical model. The total mean deviations were lower than 5 % for the proposed and validated model.

Modelling borehole flows from Distributed Temperature Sensing data to monitor groundwater dynamics in fractured media

Fractured aquifers are known to be very heterogeneous with complex flow path geometries. Their characterization and monitoring remain challenging despite the importance to better understand their behavior at all spatial and temporal scales. Heat and correspondingly temperature data have gained much interest in recent years and are often used as a tracer for characterizing groundwater flows. In the current work, a fast computer code is developed using Ramey and Hassan and Kabir analytical solutions which converts the temperature profile to the flow rate profile along the borehole. The method developed is validated through numerical simulations. A global sensitivity study recognizes the media thermal properties as the most influential parameters. For testing the method in the field, fiber-optic distributed temperature sensing (FO-DTS) data were used to monitor the dynamic behavior of fractured aquifers at the borehole scale at the Ploemeur-Guidel field site in Brittany, France. DTS data are used to infer the flow rates in the different sections of a fractured wellbore (flow profile) and calculate the contribution of each fracture to the total flow. DTS data were acquired for about three days in three different hydraulic conditions corresponding to two different ambient flow conditions and one pumping condition. Flow profiling using distributed temperature data matches satisfactorily with results from heat-pulse flow metering performed in parallel for cross-checking. Moreover, flow profiling reveals the daily variations of ambient flow in this fractured borehole. Furthermore, it shows that during ambient flowing conditions, shallow and deep fractures contribute roughly equally to the total flow while during the pumping condition, the deepest fractures contribute more to the total flow, suggesting a possible reorganization of flow and hydraulic heads depending on the hydraulic conditions. Thus, although the proposed method (DTS data and proposed framework) may be costlier and is based on indirect characterization through temperature measurements, it provides real-time monitoring of complex fracture interactions and recharge processes in fractured media. Thus, this method allows for a full analysis of the temporal behavior of the system with a simple and fast analytical model. Furthermore, thanks to its narrow width, DTS can be used and installed in boreholes for long-term monitoring while heat-pulse flow metering may lead to head losses in the borehole and may not be always possible depending on some borehole conditions. One of the limitations of the approach proposed is the proper knowledge of the thermal properties of media required to infer the flow rate from the temperature. Nevertheless, surface rate measurement can be useful to constrain these properties and reduce the flow profiling uncertainty. Thus, the method proposed appears to be an interesting and complementary method for characterizing borehole flows and groundwater dynamics in fractured media such as for instance, monitoring the recharge dynamic.

Experimental Study on the Temporal and Spatial Variation Law of Gas Flow in Different Rock Pillar Drainage Boreholes

In order to study the temporal and spatial variation of gas flow in different rock pillar extraction boreholes, gas pressure, gas content, gas emission from 100-meter coal holes and coal seam permeability coefficient were measured on site. The site inspected the gas flow in the boreholes at 15m, 7.5m, directly above, 7.5m, and 15m at the lower slab of different rock pillar floor roadways. It analyzed the change law of gas flow in boreholes of 710 floor lanes and 505 floor lanes, which provided a basis for the layout of gas drainage boreholes.

Flow Characteristics of UBD Applied to Gas Reservoirs

Underbalanced drilling technology implemented in high pressure and low permeability gas reservoirs can better protect gas formation from drilling fluid damage, and can improve the drilling speed, but gas invade into the wellhole from the formation by the underbalanced pressure will make the borehole flow presented two-phase flow state. The gas phase compactness and slippage effect will increase the drilling of well control difficulty. In order to ensure the invaded gas is not appear violent slippage and rapid inflation, study the two-phase flow law is necessary to determine the size of the applied well backpressure, applying process of gas distribution law in borehole, and the bottom hole pressure, for the reasonable design of drilling fluid density, well backpressure design and the equipment selection. This paper applies the numerical difference method to two-phase flow one-dimensional equations were solved. Combined with an example of drilling, the gas void fraction distribution, migration and bottom-hole pressure change law is given, which provides theoretical basis for low permeability gas reservoirs in underbalanced pressure drilling borehole designing.