Understanding Shale Petrophysics Helps in Drilling Stable Boreholes: Case Studies

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1 Understing Shale Petrophysics Helps in Drilling Stable Boreholes: Case Studies Mohammed K. Al-Arfaj Hamad S. Al-Kharra a ABSTRACT Drilling stable boreholes in shale formations can be a very challenging task when water-based drilling fluids are used. The interactions between the water phase in drilling fluids with shale rocks cause different drilling problems, such as shale swelling dispersion. The extent to which a shale sample swells or disperses depends on the clay content of the shale. Some types of clay have a tendency to swell more while others have a tendency to disperse more. Therefore, identifying the type of clay present in a shale rock is very important for developing drilling fluids capable of mitigating shale drilling problems. Gamma ray is a powerful tool to predict the clay mineralogy, determine shale reactivity, as a result, predict the rock response when contacted by drilling fluids. Using petrophysical properties to characterize shale can help determine the appropriate drilling fluid shale inhibitor to be used. The spectral gamma ray (GR) log has been used to determine the proportion of the radiation coming from potassium, uranium thorium. The ratio of potassium/thorium (K/Th) is utilized to identify clay mineralogy for different shale formations. In addition, formation density log was utilized with the neutron log to give more accurate information concerning the lithology. The objective of this study is to discuss how petrophysics of shale formations can be utilized to improve optimize drilling practices to achieve the ultimate goal of enhanced wellbore stability. Depending on clay content, different shale formations have different responses when exposed to drilling fluids, therefore, it is of high importance to characterize the shale formation to develop the appropriate drilling fluid. INTRODUCTION Wellbore instability is a major source of nonproductive time in drilling operations, especially when drilling shale formations. Due to the high clay content in shale rocks, they have a high level of reactivity with water-based drilling fluids. Therefore, oil-based drilling fluids have been used to drill stable boreholes in shale formations. Because of environmental regulations continuous efforts to reduce well construction costs, different operators have been shifting back from oil-based muds to water-based muds (WBMs) with an inhibition capability to drill shale formations with variable rates of success. For inhibitive WBMs, different shale inhibitors have been used to inhibit stabilize the shale against swelling dispersion. To optimize the shale inhibitor selection, shale characterization should be carried out, which includes but is not limited to mineralogy determination, the potential for hydration swelling, pore structure study, membrane efficiency estimation. Shale content has been examined in previous studies using the gamma ray (GR) log. That was through the estimation of fractional shale volume based on the minimum maximum GR values 1-4. VV ssh = GGGG llllll GGGG mmmmmm GGRR mmmmmm GGGG mmmmmm (1) where GR log is the GR reading at specific depth, GR min is the minimum GR reading GR max is the maximum GR reading. A spectral GR gives the gamma radiation of three elements: (1) potassium, () thorium, (3) uranium. Based on crossplots of potassium thorium, the clay types present can be predicted. High values indicate the presence of chlorite kaolinite, while lower values indicate the presence of montmorillonite (smectite) illite. This is a useful tool to predict the mineralogy of the shale rock compare it to the results from X-ray diffraction. Due to the use of a spectral GR to estimate the concentration of cations, it can be utilized to estimate the cation exchange capacity (CEC) 5-8. The CEC was found to correlate almost linearly with the natural GR, especially the radioactive uranium content. The formation density log is utilized to measure the bulk density of the formation in situ. The density, along with sonic log transit time, can be used to estimate the porosity, also, have some indication about the lithology. It was found that the density has a linear relationship with the logarithm of velocity for shales 9, 10. The neutron log is used to determine the porosity of the formation through the estimation of hydrogen atoms present. There are mainly three types: (1) GR/neutron tool, () side-

2 wall neutron porosity tool, (3) compensated neutron log. The neutron log can also be used to help in lithology determination. The concept is based again on the hydrogen atoms content. Unrealistic high apparent porosities from the neutron log is indicative of shales. This is a direct result of the hydrogen atoms present in bound water. The combination of density neutron logs can be used to better identify the shaly formations. When both logs are plotted simultaneously, it can be observed that they can superimpose each other for some formations while having positive or negative separation for other types of formations. For shales, due to the high apparent porosity that result from the neutron log as previously discussed, there is a positive separation where both the density neutron logs give high values. The positive separation increases with the increase in shale content. Negative separation is indicative of a clean formation such as pure sstone. The amount of separation can be used to better quantify the shale volume as follows: VV ssh = SSSSSS llllll SSSSSS ssssssss SSSSSS sshaaaaaa SSSSSS ssssssss () It must be noted that the quality of logging is affected by the borehole quality drilling fluid type. If washouts exist, the distance between the logging tool the borehole walls increases. As a result, the volume of drilling fluid increases that might affect the attenuation of the GR emitted by the formation. Also, as the density of the drilling fluid increases, there is more interference with the GR produced by the rock. Therefore, there are correction charts to correct for these effects. METHODOLOGY In this study, logging sets for the shale sections of three wells located in different fields were collected, processed, analyzed. Logging included total GR, spectral GR, the volume of clay, density, neutron logs. A total GR was used as an initial indicator of the shaleness of the formation as high GR values indicate the presence of shale. A spectral GR was utilized to predict the types of clays present based on the estimated concentrations of potassium thorium elements. Formation density neutron were analyzed to identify the shale beds provide information about the porosity. Based on the results out of this study, shale sections can be accurately characterized in terms of the mineralogy clay content. As a result, drilling fluids with specific properties additives can be used to drill stable boreholes. DATA Presented in this section are the logging for the three wells Wells A, B, C. First, Figs. 1 through 3 are the GR readings associated with volume of clay plotted against depth. Second, Figs. 4 through 6 show the spectral GR Gamma Ray (API) Clay Volume Gamma Ray Clay Volume 1. GR reading volume of clay for Well-A, yellow line = s line, red line = shale line. Gamma Ray (API) Clay Volume Gamma Ray Clay Volume GR GR reading volume of clay for Well-B, yellow line = s line, red red line = shale line = shale line. Gamma Ray (API) Clay Volume Gamma Ray Clay Volume GR GR reading reading volume volume of of clay clay for for Well-C, Well-C, yellow yellow line line = s s line, line, red line = shal red line = shale line.

3 on cross-plots of potassium thorium to help identify the types of clays. Finally, Figs. 7 through 9 are simultaneous plots for Wells A, B, C of the density neutron logging to help identify the distinct shale beds. ANALYSIS AND DISCUSSION Bulk (g/cm3) Bulk (g/cm3) In general, high GR intensities the presence of shale or Gamma Ray indicate Clay Volume shaly beds. The GR is a measure of the total natural GR radi 3. GR reading volume of clay for Well-C, yellow line = s line, red line = shale line. ation of the formation. The area located left to the GR line is indicative of the amount of detected radioactive elements. Clay Volume +Smectite Thorium (ppm) Porosity Porosity Index Index neutron neutronlogging Well-A. neutron logging for forwell-a. Bulk (g/cm3) Potassium 4.4. oncross-plot GRGR on the for Well-A. for Well-A. Thorium Thorium(ppm) (ppm) +Smectite+ +Smectite Potassium Potassium +Smectite+ +Smectite+ Thorium Thorium(ppm) (ppm) Potassium Potassium neutron logging for Well-B. GR on of for 5. oncross-plot 5.5. GRGR on the the cross-plot thorium potassium for Well-B. Well-B. for Well-B Index Porosity Porosity Index 6.6. GR oncross-plot of 6. GR GR on on the thorium potassium for Well-C. 8. neutron logging for Well-B. 8. neutron logging for Well-B. The GR for Well-A starts with moderate values that increase sharply at mid-interval again at deeper points with very high peaks of GR intensity. The very high peak toward the end of the log indicates the presence of a thin shale bed. For Well-B, it starts from a low GR value, indicating the presence of s while there is a sharp increase in GR intensity when moving a little deeper. The higher values of GR reveal the presence of shales or shaly s formations. Moving deeper, a peak of GR can be observed at the bottom of the formation where the shaliness increases even further. This can be recognized as a pure shale streak. The log for Well-C shows high intensity at intermediate depths, suggesting the presence of a shale-rich streak across the formation. Then, GRs start to decrease going deeper in the formation. The calculated shale volume was determined using Eqn. 1. SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 018

4 (ft) Bulk Bulk (g/cm (g/cm 3 ) 3 ) (ft) Porosity Porosity Index Index neutron neutron logging logging It was plotted on the same GR log to observe the correlation. To be able to calculate shale volume, the s shale lines were drawn to determine the two extremes in GR response. The two lines have been used to calculate the shale volume. Since the only variable in Eqn. 1 is the GR value at a specific depth, the correlation showed absolute agreement. In Figs. 4 through 6, s are useful tools to identify the clay types. They are basically plots of the thorium content against potassium content. High values of the ratio to potassium indicate the presence of chlorite kaolinite, intermediate values indicate the presence of smectite (montmorilloinite) mixed layers, while low values indicate the presence of illite -rich minerals. Based on the spectral GR, the three wells have high percentages of smectite mixed layers of smectite, kaolinite, illite. Well-A has relatively higher amounts of kaolinite as it has higher values of the potassium/thorium (K/Th) ratio compared to the other wells. Well-C, with its lower values of K/Th ratio, in general, has more concentrations of illite compared to other clay minerals. neutron were plotted simultaneously to be able to establish the shale s beds. Shale beds are indicated by the high values of both density neutrons. While the identification of the shale/s beds is relatively easy for Wells A B, it is more difficult for Well-C where the separation between the density neutron in interchanging. Well-A has two main beds; shale at the top s at the bottom. At the very end of the log, there seems to be another shaley bed. Well-B has one shale bed from the top to the bottom with some intervals showing more shaliness than others. Well-C has five beds interchanging between shale s. Due to the thin beds it has, it may be more convenient to look at them as one bed of shaly sstone formation especially for beds through 5. Predicting the presence of shale beds the type of clays to be encountered using the logging of the offset wells helps in selecting the appropriate type of drilling fluid the optimum orientation. The shale petrophysical provides a complete picture of the formation s mineral content properties. Optimization of the drilling fluid to be used starts with knowing what the minerals to be encountered are. Then, the expected interactions between the rock minerals the drilling fluids are accounted for methods to mitigate the negative impact are implemented. For example, for a shale rock with high smectite content, the rock has a high tendency to swell. Therefore, swelling inhibitors can be added to the drilling fluids to minimize the scope of swelling, then, wellbore instability. In general, the shale reactivity increases with the increase in clay content. In addition, the types of clays present in the shale samples give different types of problems depending on several factors such as CEC capillary suction. has low CEC, but is a dispersion type of clay. When exposed to water, it disintegrates disperses causes the shale fabric to become weak loose. In the wellbore, this effect causes shale formation heaving sloughing problems, in severe cases may lead to hole collapse. Different shale additives, such as long-term polyamine shale inhibitors, can be used to mitigate the long-term as well as short-term problems due to shale-fluid interactions. has low to intermediate CEC values depending on the impurities. Therefore, it has a low tendency to swell, at the same time, it is not a dispersive type of clay. Therefore, a shale sample with high illite content may not need excessive amounts of shale stabilizers. It is important to mention that illite exists in mixed layers with smectite. This is mainly due to transformation of smectite illite over geologic periods of time. In the transformation period before the illite transforms completely to smectite, it exists in the mixed layer state. The illite-smectite mixed layer is a well-known frequently encountered type of clay. It has moderate to high CEC, therefore, is a swelling clay behaves similar to smectite. Next comes the highest swelling clay; montmorillonite, which falls under the smectite category. This type of clay has high values of CEC, especially when it is present as sodium montmorillonite. This type of clay can swell exp to more than double the original volume with distilled water. Swelling clays are considered the major clay types that cause drilling problems due to the wellbore instability they cause. The expansion in clay volume weakens the shale matrix

5 makes it susceptible to caving in washouts. There is even experimental evidence 11 that as swelling clay starts to increase in volume upon contact with water, there is a threshold value of expansion after which the clay particles start to breakdown on the molecular level. This is due to the mechanical changes taking place as water molecules are introduced in the clay system. CONCLUSIONS In this article, the petrophysical of three wells has been acquired processed in an attempt to better characterize the shale section. This was accomplished using from the total GR, spectral GR, density, neutron logs. The results showed the high percentage of smectite present in the shale section of these wells. After smectite come the kaolinite, illite, the mixed layer of these three clay minerals. Shale volume was calculated showed correlation with the GR. Shale beds have been identified based on the combined density neutron. Two wells have thick shale beds while the third one has a thick bed of shaly s. Characterization of shale formations using the most accurate techniques workflows can have a positive impact to enhance wellbore stability reduce nonproductive time. Based on the results of processing different types of petrophysical, best practices can be developed for more efficient drilling performance. ACKNOWLEDGMENTS The authors would like to thank the management of Saudi Aramco for their support permission to publish this article. This article was presented at the SPE/IATMI Asia Pacific Oil Gas Conference Exhibition, Bali, Indonesia, October 17-19, Juhasz, I.: Normalized Qv The Key to Shaly S Evaluation Using the Waxman-Smits Equation in the Absence of Core Data, SPWLA paper 1981-Z, presented at the SPWLA nd Annual Logging Symposium, Mexico City, Mexico, June 3-6, Atwater, J.E.: Correlation of Cation Exchange Capacity with Core Gamma Ray Logs, SPWLA paper 1986-QQ, presented at the SPWLA 7 th Annual Logging Symposium, Houston, Texas, June 9-13, Ramirez, M.O.: Cation Capacity Exchange Data Derived from Well Logs, SPE paper 1097, presented at the SPE Latin American Petroleum Engineering Conference, Rio De Janeiro, Brazil, October 14-19, Mian, M.A. Hilchie, D.W.: Comparison of Results from Three Cation Exchange Capacity Analysis Techniques, The Log Analyst, Vol. 3, Issue 5, September Alger, R.P., Raymer Jr., L.L., Hoyle, W.R. Tixier, M.P.: Formation Log Applications in Liquid Filled Holes, Journal of Petroleum Technology, Vol. 15, Issue 3, March 1963, pp Gardner, J.S. Dumanoir, J.L.: Litho- Log Interpretation, SPWLA paper 1980-N, presented at the SPWLA 1 st Annual Logging Symposium, Lafayette, Louisiana, July 8-11, Katti, D.R., Matar, M.I., Katti, K.S. Amar, P.M.: Multiscale Modeling of Swelling Clays: A Computational Experimental Approach, KSCE Journal of Civil Engineering, Vol. 13, Issue 4, July 009, pp REFERENCES 1. Alguero, A.J., Fabris, A., Watt, H.B. Wichmann, P.A.: Effective Porosity Shale Volume Determination from Lifetime Sidewall Logs, SPWLA paper 1973-X, presented at the SPWLA 14 th Annual Logging Symposium, Lafayette, Louisiana, May 6-9, Heslop, A.: Gamma Ray Log Response of Shaly Sstones, SPWLA paper 1974-M, presented at the SPWLA 15 th Annual Logging Symposium, McAllen, Texas, June -5, Bhuyan, K. Passey, Q.R.: Clay Estimation from GR Porosity Logs, SPWLA paper 1994-DDD, presented at the SPWLA 35 th Annual Logging Symposium, Tulsa, Oklahoma, June 19-, Katahara, K.W.: Gamma Ray Log Response in Shaly Ss, The Log Analyst, Vol. 36, Issue 4, July 1995.

6 BIOGRAPHIES Mohammed K. Al-Arfaj joined Saudi Aramco in 006 as a Petroleum Engineer, working with the Drilling Technology Team in the Exploration Petroleum Engineering Center Advanced Research Center (EXPEC ARC). He works in the area of drilling completion, has conducted several projects in the areas of shale inhibition, drilling nano-fluids, loss circulation materials, spotting fluids, swellable packers, completion fluids, oil well cementing. Mohammed received his B.S. degree in Chemical Engineering from King Fahd University of Petroleum Minerals (KFUPM), Dhahran, Saudi Arabia, in 006. In 009, he received his M.S. degree in Petroleum Engineering from Heriot-Watt University, Edinburgh, Scotl. In 017, Mohammed received his Ph.D. degree in Petroleum Engineering, specializing in molecular modeling experimental studies of shale-fluid interactions, from KFUPM. Hamad S. Al-Kharra a joined Saudi Aramco in July 008 as a Petroleum Engineer, is currently working with the North Arabian Strategy team of the Unconventional Gas Asset Department. His work experience includes working on the field development plan for the Marjan field. Hamad also worked with the Reservoir Description Division for almost 18 months, working on operation production (Quicklook visual analysis formation analysis log). After that, he went on an out-of-kingdom assignment for training in unconventional gas reservoir in Houston, TX. In 010, Hamad received his B.S. degree in Petroleum Engineering from King Fahd University of Petroleum Minerals (KFUPM), Dhahran, Saudi Arabia. He then pursued a M.S. degree in Petroleum Engineering on a part-time basis from KFUPM, receiving it in March 013. Hamad is also a Society of Petroleum Engineers (SPE) certified Petroleum Professional.