A fast forward algorithm for real-time geosteering of azimuthal gamma-ray logging

doi:10.1016/j.apradiso.2017.02.042

Applied Radiation and Isotopes

Volume 123, May 2017, Pages 114-120

https://doi.org/10.1016/j.apradiso.2017.02.042 Get rights and content

Highlights

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A fast forward algorithm is proposed to geosteer using azimuthal gamma-ray logging.
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A simple method is proposed to determine the formation parameters of the algorithm.
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Distance from logging tool to boundary can be obtained using the forward method.

Abstract

Geosteering is an effective method to increase the reservoir drilling rate in horizontal wells. Based on the features of an azimuthal gamma-ray logging tool and strata spatial location, a fast forward calculation method of azimuthal gamma-ray logging is deduced by using the natural gamma ray distribution equation in formation. The response characteristics of azimuthal gamma-ray logging while drilling in the layered formation models with different thickness and position are simulated and summarized by using the method. The result indicates that the method calculates quickly, and when the tool nears a boundary, the method can be used to identify the boundary and determine the distance from the logging tool to the boundary in time. Additionally, the formation parameters of the algorithm in the field can be determined after a simple method is proposed based on the information of an offset well. Therefore, the forward method can be used for geosteering in the field. A field example validates that the forward method can be used to determine the distance from the azimuthal gamma-ray logging tool to the boundary for geosteering in real-time.

Introduction

Due to better production and economic benefits in complex and unconventional reservoirs, horizontal well technologies are widely used (Meehan, 1994, Meador, 2009). Before drilling horizontal wells, the pre-drilling strata model is usually established. However, because of few references and complicated geological conditions, the pre-drilling strata model has a certain degree of uncertainty (Zhu et al., 2013). Therefore, real time geosteering is needed to ensure the high reservoir drilling rate and considerable economic benefits (Chen et al., 2014).

Geosteering usually used information measured by logging while drilling (LWD). Early LWD still used conventional logging methods with radial average measurement that was suitable for vertical well. These methods can not distinguish between the upper and bottom boundary when drilling horizontal wells in a reservoir. Therefore, LWD azimuthal resistivity logging and azimuthal gamma-ray logging have been developed. Their measurement contains the direction information, and can be inverted to obtain the upper or bottom boundary information in reservoir (Su, 2005). Azimuthal resistivity tool can provide a key measurement designated as the geosignal (Bittar, 2002, Bittar et al., 2009, Bittar et al., 2010). One of benefits of the geosignal is sensitive to only lateral variations in resistivity and can detect various investigation depths (farthest 18 ft), and the other important property is a near-exponential dependence of the geosignal on the distance to bed boundary (Seifert et al., 2009, Chemali et al., 2008, Chemali et al., 2009). Thus it facilitates proactive prediction of boundaries (Harris et al., 2009, Diaz et al., 2009). Combined the geosignal, azimuthal deep and shallow resistivities are superior in locating bed boundaries to keep the well in the desired pay zone for optimal drainage (Bittar et al., 2009, Seifert et al., 2009). In most cases, resistivity methods have been the premier choice for wellbore placement in geosteering. In some cases, various measurements, e.g. azimuthal gamma-ray and density logging, may be complementary, such as when the resistivity contrast of the near- and far-beds is low and/or the reservoir is thin (Palmer et al., 2008, Calleja et al., 2010, Seifert et al., 2011). In a thin reservoir within a complex geological environment, the conjunction of azimuthal gamma and resistivity can help the wellbore enter the thin reservoir and remain within it for an extended reach (Pitcher et al., 2009). In the thin target zone (10-ft-thick) of a complex unconventional reservoir, 85% of the well length was navigated with azimuthal gamma only (Martin et al., 2013). Thus in low resistivity contrast and/or thin reservoirs, azimuthal gamma can be better used to geosteer by combining with azimuthal resistivity, and the two measurements are complementary (Mohamed and Jeremy, 2014). When using azimuthal gamma-ray, the up and bottom gamma-ray logs (GRU and GRB) measured by the tool usually can be used to geosteer (Mottahedeh, 2008). Currently, the representative tools of azimuthal gamma-ray logging while drilling are sondes of EcoScope from Schlumberger (Neville et al., 2007), OnTrak from Baker Hughes (Efnik et al., 1999), GABI from Halliburton (Pitcher et al., 2009) and SAGR from Weatherford (Mickael et al., 2002).

Many mathematical methods have been used to study the response of radioactive logging. Wu and Huang (2004) used the Monte Carlo method to analyze the factors that influence and optimize the structure parameters of the radioactive logging tool. Dworak et al. (2011) proposed a numerical modeling to analyze the responses of a gamma-gamma density tool by using the Monte Carlo method. Considering the calculation speed of Monte Carlo, the method is limited because of its huge amount of data and the real time geosteering required making decision in a short period of time (Phillips et al., 2000); some researchers have tried to study fast forward method of gamma-ray logging in the published literature. Shao et al. (2013) proposed the fast forward model for the LWD gamma-ray logging without azimuthal information, and it can be applied to geosteer in conventional reservoirs. Based on the numerical simulation results of Monte Carlo method, Yuan et al. (2015) proposed a novel method to calculate the distance from the drilling bit to the boundary when the drilling bit passes through the boundary. However, when drilling bit nears boundary, the distance from it to the boundary should be predicted for better geosteering. Thus further studies of the forward method of azimuthal gamma-ray logging should be carried out.

In this paper, based on the structural characteristics of an azimuthal gamma-ray logging tool, the detecting spatial scope characteristics of logging tools are summarized in horizontal wells. In different parts of detecting spatial scope of probe in azimuthal gamma-ray tool, the corresponding integral calculation model is deduced, and the forward algorithm of azimuthal gamma-ray logging in different spatial positions is set up. By using the forward method, the response characteristics of azimuthal gamma-ray logging are simulated, and the simulation result indicates that the method can be used to determine the distance from the logging tool to the boundary when it nears the boundary. After the formation parameters of the method are determined, the method is applied in a horizontal well, and the reliable result can confirm its availability and prospects in geosteering.

Section snippets

Scaling of azimuthal gamma logging tool

In the measurement principle, LWD azimuthal gamma logging and conventional gamma logging are roughly the same. They usually use a gamma scintillation counter to detect the natural gamma ray from different radioactive elements (K⁴⁰, Th and U) in strata. A numerical model of the azimuthal gamma logging tool is used in the simulations as is shown in Fig. 1. The LWD azimuthal gamma logging tool is usually installed with 4 probes in each slot. The probe (the NaI gamma-ray detector) can detect only

Background information

While LWD logging, the azimuthal gamma tool usually has two working modes, one is that the azimuthal gamma short piece rotates together with the drilling system; the other is to keep a certain angle not with the rotation system. Most often the first mode is recommended, because the second model does not maintain a steady angle but drifts slowly (Pitcher et al., 2009; Mohamed and Jeremy, 2014). Regardless of which model is in operation, they can measure the GRU and GRB come from the upper and

Numerical model of strata medium

The strata model is a layered, homogeneous medium. From top to bottom, in turn, there are parallel layer A, layer B (the middle layer, thickness of H), and layer C. Considering the relationship between the horizontal well path and the space position of the layered strata and the spatial characteristics of instrument investigation range (Pitcher et al., 2009; Mohamed and Jeremy, 2014), Fig. 3 shows the final three types of geometry used in the calculations: (1) Fig. 3a shows that layer A and

Numerical simulation model

In the infinite homogeneous layer, gamma-ray flux in the observation point with radius of R can be calculated by (Huang, 1985) $d J = \frac{a q ρ}{4 π r^{2}} e^{- u r} d v = \frac{a q ρ}{4 π} \sin θ \cdot e^{- u r} d r d θ d φ$ where, J denotes gamma ray flux in the observation point, s⁻¹ cm⁻²; ρ is formation density, g cm⁻³; q denotes radioactive substances contained in each gram of rock, g g⁻¹; a represents the number of gamma photons emitted per gram of radioactive material per second, g⁻¹ s⁻¹; μ denotes the absorption coefficient of formation for gamma

Calculations results and interpretation

The lithology of the homogenous stratum model is sandstone and shale, and they are respectively representative of LRR (low-level radioactive reservoir) and HRNR (high-level radioactive non-reservoir), and their radioactive and petrophysical parameters are listed in Table 1. The average number of gamma photons emitted per second of K⁴⁰ is 3.4, Th is 1.0×10⁴, and U is 2.8×10⁴, respectively. Therefore, the gamma ray flux of GRU (or GRB), respectively, are 0.061149 and 0.68358 in sandstone layer

Field application

In oil field production, the formation parameters used in the forward model can be determined by experiments. But it is not convenient and economic for technological factors. For real-time geosteering in this paper, a simple method is proposed to determine the parameters of strata rocks. There are usually some vertical wells around the horizontal well. The response of the target layer in the vertical well can be calculated (Huang, 1985): $J_{0} = \frac{a q p}{μ}$

Note that in the same layer, the parameters a, q, ρ

Conclusion and discussion

Due to complicated geological conditions, geosteering while drilling is needed. In the low resistivity contrast and/or thin layer formation, combined with resistivity logging, azimuthal gamma-ray logging can be used to obtain boundary information for geosteering.

Based on natural gamma ray distribution equation in formation, the forward algorithm of azimuthal gamma-ray logging is deduced. Simulation result indicates that the forward method calculates fast, and when the well path nears the

Future work

Considering the logging response of azimuthal gamma-ray logging is sensitive to the boundary, the timely corrections of the well trajectory will be frequent and troublesome in geosteering. Thus a method or program, like the real-time inversion software of azimuthal resistivity data (Chemali et al., 2009), needs further study to reduce manual workload for application and dissemination. In addition, the response of other geologic bodies, e.g. fault and a lens, needs further studied.

Acknowledgements

We sincerely acknowledge the anonymous reviewers whose correlations and comments have greatly improved the manuscript. The authors are thankful to the National Natural Science Foundation of China (No. 41074086), the Science and Technology Research Projects of Hubei Provincial Department of Education (D20141301) and the Scientific Research and Technology Development Project of China National Petroleum Corporation (2013E-3809) for their generous support.

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