A Transient-Pressure-Based Numerical Approach for Interlayer Identification in Sand Reservoirs

Almost all sandstone reservoirs contain interlayers. The identification and characterization of these interlayers iscritical for minimizing the uncertainty associated with oilfield development and improving oil and gas recovery.Identifying interlayers outside wells using identification methods based on logging data and machine learning isdifficult and seismic-based identification techniques are expensive. Herein, a numerical model based on seepageand well-testing theories is introduced to identify interlayers using transient pressure data. The proposed modelrelies on the open-source MATLAB Reservoir Simulation Toolbox. The effects of the interlayer thickness, position,and width on the pressure response are thoroughly investigated. A procedure for inverting interlayer parametersin the reservoir using the bottom-hole pressure is also proposed. This method uses only transient pressuredata during well testing and can effectively identify the interlayer distribution near the wellbore at an extremelylow cost. The reliability of the model is verified using effective oilfield examples.

Research on Pore Structure and Fractal Characteristics of Tight Sandstone Reservoir Based on High-Pressure Mercury Injection Method

Pore structure is the key element of tight sandstone reservoir, which restricts the accumulation and flow of oil and gas in the reservoir. At present, reservoir pore structure is the focus and difficulty of unconventional oil and gas exploration and development research. The tight sandstone reservoir in the Chang 4 + 5 member of the Upper Triassic Yanchang Formation is the main reservoir for oil and gas exploration in G area. At present, there is little research on its pore structure and fractal characteristics, which to some extent affects the progress of exploration and development. This paper selects the tight core samples of the Chang 4 + 5 member in the southern edge of the Ordos Basin, and based on the high-pressure mercury intrusion experiment, uses fractal theory to study the pore structure and fractal characteristics of the reservoir in the study area, thus providing theoretical basis for the evaluation and exploration and development of the Chang 4 + 5 tight reservoir in the G area. The research results show that the lithology of the Chang 4 + 5 tight sandstone reservoir in the southern edge of the Ordos Basin is mainly feldspathic sandstone, with the highest feldspar content, followed by quartz, and the clay mineral is mainly chlorite. The reservoir has poor physical properties and strong heterogeneity. There are three main fractal characteristics in Chang 4 + 5 reservoir in G area: the fractal curve of Type I reservoir sample is in two segments, the relatively large pore has certain fractal characteristics, the pore structure is relatively regular, and the heterogeneity is weak;Relatively small pores have no fractal characteristics and pore structure is irregular. The fractal curve of Type II reservoir samples shows a three-segment pattern, and each pore size range has certain fractal characteristics, and it gradually gets better with the increase of pore size. The fractal curve of Type III reservoir samples presents a similar one-segment pattern, and the fractal dimension exceeds the upper limit of 3. It is considered that the full pore size of this type of reservoir does not have fractal characteristics, the pore throat is completely irregular or the surface is rough, and the heterogeneity is very strong.

Main flow channel index in porous sand reservoirs and its application

Based on well test interpretation,production performance analysis,overburden permeability and porosity test,gas-water core flooding test and high-pressure mercury injection,a quantitative correlation has been built of in-situ effective permeability with routine permeability and water saturation,and the ranges of Main Flow Channel Index(MFCI)are determined for different permeability levels in porous sand gas reservoirs.A new method to evaluate the in-situ effective permeability of porous sand reservoir and a correlation chart of reserves producing degree and main flow channel index are established.The results reveal that the main flow channel index of porous sand gas reservoirs has close correlation with routine matrix permeability and water saturation.The lower the routine matrix permeability and the higher the water saturation,the lower the MFCI is.If the routine matrix permeability is greater than 5.0×10-3,the MFCI is generally greater than 0.5.When the routine matrix permeability is from 1.0×10-3 to 5.0×10-3,the MFCI is mainly between 0.2 and 0.5.When the routine matrix permeability is less than 1.0×10-3,the MFCI is less than 0.2.The evaluation method of in-situ effective permeability can be used to evaluate newly discovered or not tested porous sand gas reservoirs quickly and identify whether there is tight sand gas.The correlation chart of reserves producing degree and main flow channel index can provide basis for recoverable reserves evaluation and well infilling,and provide technical support for formulation of reasonable technical policy of gas reservoir.

Assessment of the Spectral Decomposition Techniques in the Evaluation of Hydrocarbon Potential of “BOMS” Field, Coastal Swamp Niger Delta, Nigeria

This study employs the different approaches of the spectral decomposition techniques to evaluate the hydrocarbon potential of the reservoir and analyse to determine the most efficient spectral decomposition technique with better resolution using the “BOMS” Field, coastal swamp depobelt Niger Delta, Nigeria. A good number of drilled wells have failed both in the Niger Delta Basin and other basins due to a poor understanding of the reservoir properties in advance of drilling and identifying the best approach will help to minimize this risk. Seismic and well logs data together with the Hampson Russel 10.3 software were used for the study. The target reservoirs were identified from the suite of well logs at the horizons with low gamma ray, high resistivity, and low acoustic impedance between TVD (ft) of 10,350 - 10,450 ft. The analysis of the amplitude spectrum of the seismic data revealed that the distortion of interest lies between 5 - 60 Hz. Seismic data were then spectrally decomposed into several frequencies such as low frequency (15 Hz), mid-frequency (31 Hz), and high frequency (46 Hz) where distortions were observed. Time- frequency slices of 15 Hz and 23 Hz provided clearer events (potential hydrocarbon sand) indicated by high amplitude envelope (2200 - 2400) and amplitude anomalies. While the amplitude dropped in the mid-frequency (31 Hz), the high amplitude envelope and the high energy completely disappeared in the high (46 Hz) time-frequency slice. A comparison of the Short- time Fourier transform and the Basic Pursuit algorithm revealed that the Basic Pursuit provided a better resolution of the reservoir characteristics than the former. The Red, Green and Blue (RGB) colour blending model indicated that the channel was consistent with the low-frequency section and amplitude anomaly.

Tight gas production model considering TPG as a function of pore pressure,permeability and water saturation

Threshold pressure gradient has great importance in efficient tight gas field development as well as for research and laboratory experiments.This experimental study is carried out to investigate the threshold pressure gradient in detail.Experiments are carried out with and without back pressure so that the effect of pore pressure on threshold pressure gradient may be observed.The trend of increasing or decreasing the threshold pressure gradient is totally opposite in the cases of considering and not considering the pore pressure.The results demonstrate that the pore pressure of tight gas reservoirs has great influence on threshold pressure gradient.The effects of other parameters like permeability and water saturation,in the presence of pore pressure,on threshold pressure gradient are also examined which show that the threshold pressure gradient increases with either a decrease in permeability or an increase in water saturation.Two new correlations of threshold pressure gradient on the basis of pore pressure and permeability,and pore pressure and water saturation,are also introduced.Based on these equations,new models for tight gas production are proposed.The gas slip correction factor is also considered during derivation of this proposed tight gas production models.Inflow performance relationship curves based on these proposed models show that production rates and absolute open flow potential are always be overestimated while ignoring the threshold pressure gradients.

Prediction of rock brittleness using nondestructive methods for hard rock tunneling

Sand production is an undesired phenomenon occurring in unconsolidated formations due to shear failure and hydrodynamic forces. There have been many approaches developed to predict sand production and prevent it by changing drilling or production strategies. However, assumptions involved in these approaches have limited their applications to very specific scenarios. In this paper, an elliptical model based on the borehole shape is presented to predict the volume of sand produced during the drilling and depletion stages of oil and gas reservoirs. A shape factor parameter is introduced to estimate the changes in the geometry of the borehole as a result of shear failure. A carbonate reservoir from the south of Iran with a solid production history is used to show the application of the developed methodology. Deriving mathematical equations for determination of the shape factor based on different failure criteria indicate that the effect of the intermediate principal stress should be taken into account to achieve an accurate result. However, it should be noticed that the methodology presented can only be used when geomechanical parameters are accurately estimated prior to the production stage when using wells and field data.

A quantitative study of the scale and distribution of tight gas reservoirs in the Sulige gas field, Ordos Basin, northwest China

Gas and water distribution is discontinuous in tight gas reservoirs,and a quantitative understanding of the factors controlling the scale and distribution of effective reservoirs is important for natural gas exploration.We used geological and geophysical explanation results,dynamic and static well test data,interference well test and static pressure test to calculate the distribution and characteristics of tight gas reservoirs in the H_(8) Member of the Shihezi Formation,Sulige gas field,Ordos Basin,northwest China.Our evaluation system examines the scale,physical properties,gas-bearing properties,and other reservoir features,and results in classification of effective reservoirs into types Ⅰ,Ⅱ,and Ⅲ that differ greatly in size,porosity,permeability,and saturation.The average thickness,length,and width of type Ⅰ effective reservoirs are 2.89,808,and 598 m,respectively,and the porosity is>10.0%,permeability is>1010^(–3)µm^(2),and average gas saturation is>60%.Compared with conventional gas reservoirs,tight gas effective reservoirs are small-scale and have low gas saturation.Our results show that the scale of the sedimentary system controls the size of the dominant microfacies in which tight gas effective reservoirs form.The presence of different types of interbeds hinders the connectivity of effective sand body reservoirs.The gas source conditions and pore characteristics of the reservoirs control sand body gas filling and reservoir formation.The physical properties and structural nature of the reservoirs control gas–water separation and the gas contents of effective reservoirs.The results are beneficial for the understanding of gas reservoir distribution in the whole Ordos Basin and other similar basins worldwide.

Vertical-well-assisted SAGD dilation process in heterogeneous super-heavy oil reservoirs:Numerical simulations

PetroChina’s Karamay heavy oil reservoirs mostly comprise Jurassic deposits of braided fluvial facies that have a low porosity,extremely high bitumen viscosity,and strong heterogeneities within the reservoir.Steam-assisted gravity drainage(SAGD)dilation start-up has been proven an efficient reservoir geomechanical stimulation method as it shortens the steam circulation time,reduces steam usage,and increases the oil production rates.This method worked well on relatively good-quality oil sand reservoirs.However,for reservoirs with heterogeneous reservoir properties along the SAGD well,field well production data indicated the presence of non-uniform steam chambers in post-dilation SAGD wells.Enhanced dilation methods,such as vertical-well-assisted SAGD dilation(VW-SAGD-Dilation)are have been used,and field pilot tests have yielded promising results.This reservoir stimulation process utilizes one or more vertical wells beside the SAGD well to initiate the formation of dilation zones that connect to the dilation zone formed during the conventional SAGD dilation start-up.Compared with the normal SAGD dilation start-up,this composite dilation process creates a uniform dilation zone along the SAGD well,thus improving the thermal conformance and well productivity.The vertical well and SAGD well are connected by this composite dilation zone,which enables the combined use of the SAGD process and cyclic steam stimulation process.This paper presents our understanding and numerical case studies of this enhanced dilation strategy.The well production data from the postdilation wells were positive(uniform steam chamber growth and higher oil rate),which highlights the promising potential of using geomechanical dilation as a reservoir stimulation method to create a large stimulated reservoir volume in heterogeneous super-heavy oil reservoirs.

Control of Anticline Crest Zone on Depositional System and Its Geological Significance for Petroleum in Changshaling,Ying- er Sag,Eastern Jiuquan Basin

ABSTRACT： Yinger sag is the main petroleum and only the Changshaling oil resource has shaling anticline crest zone was formed above the generative sag in eastern Jiuquau Basin, been found after 60 years prospecting. Chang- regional uplift, and was slightly affected by crest was identified as a structural transposition zone. Based on analysis of seismic facies, logging facies and seismic inversion technique, it has been demonstrated that anticline crest zone conducts drainage entering basin and dominates sedimentary detritus further dispersing, and the anticline crest zone controls the distribution of sand bodies and the development of sedimentary system consequently. The sequence stratigraphic patterns of multi-step fault belt in Member 3 of Xiagou Formation in Lower Cretaceous in the anticline crest zone is composed of the development of sedimentary facies in plan and distribution of multi-step fault belt sand bodies in spa- tial. This research investigates the sand-control models and sequence patterns, and finally a model of reservoirs in anticline crest zone is identified.