SEPTEMBER 2018 The “Better Business” Publication Serving the Exploration / Drilling / Production Industry Approach Optimizes Well Geosteering By Christopher Viens When a change in total gamma is not based 3-D geosteering solution rather and Mark Tomlinson associated with an up or down movement than conventional 2-D geosteering soft- through stratigraphy, it indicates faulting, ware. By integrating multiple well and HOUSTON–Geosteering in horizontal a depositional anomaly, heterogeneity, or seismic surface inputs, this approach wells involves correlating logging data bedding that is not laterally continuous gives the geosteering geologist the infor- to a type log from a nearby offset well to or stratified. Because the fundamental mation to confidently resolve abrupt bed characterize the zone of interest. Corre- basis of geosteering is correlating to dip changes, identify faults, identify areas lating against a type log requires the bed- marker beds that have lateral continuity, of lateral continuity/discontinuity, identify ding thickness and gamma character of in situations where lateral continuity is stratified/unstratified zones and understand the target well to be close to that of the absent, only a low level of interpretation formation-related directional drilling tra- type log, which can be offset by miles in confidence can be achieved using tradi- jectory phenomena, etc. some cases. If not, the resulting geosteering tional correlation methods. interpretation will have unreasonable bed To maximize the benefits of azimuthal Laterally Continuous Bedding dips that do not accurately reflect the gamma imaging, a protocol has been de- In areas with laterally continuous target lithology being drilled, leaving the veloped to identify and handle challenging strata, the gamma character in the type geosteering geologist running multiple geosteering situations using a model- well typically will be present at the simultaneous interpretations in the hope that one will start to make sense as new data come in during drilling. FIGURE 1 Azimuthal gamma imaging, in con- Incorrect (Top) and Correct (Bottom) Geosteering Interpretations junction with continuous inclination meas- With Type Log (Right) urements, brings clarity to stratigraphic 1,000’ Measured Depth s ° position, bed dip, faults and zones where y 5 Type Log n e 9 o v i t r Gamma Ray bedding is laterally discontinuous or com- a a u n i 0 150 l S c m n ’ pletely absent. In many of these situations, I m 0 y a e 9 v / r G geosteering cannot be performed with u w ° l S 5 a 8 n t o traditional solutions. Through enhanced o i t T 0 a 5 t d visibility of the subsurface by azimuthal 1 y e n D r a a V R p T r a gamma imaging, geological anomalies ’ e 5 m t 1 m n a I can be identified quickly and handled in G the best possible manner. 0 Utilizing azimuthal gamma imaging n o 1,000’ Measured Depth i t ° a a 5 n 9 i data to identify faults, laterally continu- l m c s n I m u s ous/discontinuous bedding, stratified/un- a o u o u G u l ° n n i i 5 stratified zones, and high-energy sedi- t t a 8 n n h o t o C mentary deposition improves interpretation u C / m Total Gamma Ray i w z accuracy. Azimuthal gamma logging tools A n o 90’ Surveys d i measure the incoming gamma radiation t n a t a e n from all sides of the borehole and provide r Cont. Inclination o p i r t 0 e a 5 t a clear indication of whether a change in y 1 a n n i R I l c total gamma resulted from stratigraphic a n m I m a movements up or down in horizontal G drilling applications. 0 Reproduced for DynaView with permission from The American Oil & Gas Reporter www.aogr.com FIGURE 2 bit. Surface deformation is propagated ahead Bed Counting Method to Track Stratigraphic Position of the bit with a corresponding change in projected synthetic petrophysical curves. This permits greater understanding of ex - 0 1 2 1 0 (__-1__ ) -2 -3 -4 -5 -6 -5 -4 -3 -2 -3 -2 -3 -2 pected properties ahead of the bit to mitigate 0 0 ) 2 I unnecessary steering changes based on mis - P A ( interpretations, and allows smoother and a m more gradual steering adjustments to remain m a G in the target zone while mitigating increased 0 wellbore tortuosity. 1,000’ MD This continuous updating with new data and correlations results in a high-quality +2 geomodel that can be used in laterally +1 0 continuous bedding to model the azimuthal gamma response along a projected well -1 path. As real-time gamma image data are -2 logged, a close match between the synthetic -3 azimuthal gamma image and measured -4 azimuthal image is a sign of an accurate -5 stratigraphic position interpretation. -6 This projection method reduces unnec - essary course corrections and increases landing point and throughout the well - curve because of its downward-cutting correlation confidence. To achieve an ac - bore’s lateral length. This type of bedding trajectory. curate synthetic image, a 3-D model is re - is ideal for geosteering since a proper in - quired to account for bed thickening/thinning terpretation will overlay the type log with Synthetic Data Between Wells and topographical changes. A close match little deviation. While these cases are When improved imaging visibility is between both images gives the geologist a ideal for geosteering, they are still subject combined with 3-D geomodeling incorpo - high level of confidence in the interpretation to ambiguity when utilizing a 90-foot rating on-the-fly correlation and updating, and the accuracy of the geomodel. When survey and only total gamma data. a higher level of correlation confidence encountering unexpected laterally discon - tinuous bedding, the 3-D model provides Figure 1 demonstrates how geosteering can be achieved. Geomodeling permits the greater confidence in maintaining trajectory interpretations can be ambiguous and creation of synthetic petrophysical data by until correlatable laterally continuous and highly subjective as a result of the position propagating data from all nearby wells, stratified features are encountered again. of 90-foot surveys, as well as a lack of accounting for thickening and thinning ac - Since azimuthal gamma image features up or down stratigraphic movement indi - cording to the geometry of surfaces within are a result of wellbore trajectory, bed cations from the total gamma measure - the model. These synthetic properties can dip angle and wellbore direction, to ac - ment. The type log at right shows a 15- be projected through the model and com - curately generate synthetic images that foot vertical profile of a zone. The inter - pared with actual data. Correlation adjusts honor measured images, utilizing a con - pretation at the top is based on 90-foot surface depths and thicknesses, resulting tinuous inclination-generated survey results surveys and total gamma data. In contrast, in a corresponding change of character in in a more accurate match. The relationship the lower interpretation uses continuous the synthetic curves, resulting in validation between bed dip, wellbore survey and inclination and azimuthal gamma imaging. of the geosteering interpretation. gamma image features are presented in The different stratigraphic position as in - A correctly interpreted correlation is re - Figure 3, which demonstrates discrepancies dicated by the numbering is a result of flected in high character coherence between between a 90-foot survey position (left) the definitive direction indication from the actual and synthetic curves behind the azimuthal gamma, and the continuous inclination curve bringing visibility to FIGURE 3 wellbore inclinations not characterized Bed Dip Calculation Utilizing 90-foot Surveys by the 90-foot surveys. Versus Continuous Inclination Figure 1 shows only 1,000 feet of meas - ured depth in a lateral. One can imagine how many incorrect interpretations may be made over the course of a 5,000-10,000 foot lateral simply because of data ambiguity if the proper measurements for accurate wellbore placement are not utilized. In laterally continuous zones without faulting or strata pinching, the bed counting method (Figure 2) can be performed to track stratigraphic positioning relative to the landing point. This is a powerful method for validating stratigraphic posi - tioning without actively performing a geosteering correlation. However, the method is not effective for landing a Special Report: Reservoir Modeling & EUR and continuous survey position gamma Integrating well data and geological horizons graphic position, and in the event bedding image (right) for the same dip pick. In (using seismic horizons or synthetic surfaces is too chaotic for any regional or local this case, the continuous survey shows a tied to regional offsets) permits the propa - correlatable feature, the 3-D model can 2.1-degree difference in bed dip compared gation of offset information from multiple be used as a guide to maintain the well with the 90-foot survey calculation. wells to the actual drilled well location. path in the target lithology. The lack of wellbore inclination char - When encountering zones in a lateral Rapid interpretation of information acterization is one of the largest contributing wellbore where an azimuthal gamma image also is aided through data visualization factors to true vertical depth measurement identifies chaotic bedding or lateral dis - that can be extracted from a 3-D model. errors. This error presents itself in bed continuity, the 3-D model can be used to Contour maps of encountered horizons dip calculations when dip picking azimuthal project a well path ahead of the bit that will can account for lateral wellbore trajectory gamma image features, as well as across maintain the trajectory within the theoretical changes, and also assist in interpreting correlation cells in geosteering interpre - zone of interest until definable bedding or stratigraphic positional changes related tations.