Abstract: Resistivity Modeling Applied to Geosteering and Formation Evaluation in Vermelho Field, Campos Basin, Brazil.

Denicol, P. S., Meira, A. A. G., Soares, G. F. and Coutinho, M. R. -Petrobras/E&P

This paper focuses on forward modeling and resistivity simulation to predict the response of the 2Mhz electro-magnetic propagation tool in horizontal wells. The simulation of the resistivity curves is applied to geosteering in order to keep the well trajectory in target and optimize the reservoir exposure to the borehole. The behavior of the resistivity curves (Attenuation and Phase resistivities) is largely used to recognize bed boundaries and adjacent beds. The main objective of this geosteering procedure is to keep the well trajectory within the pay zone and in a safe position away from the underlying oil/water contact. The method includes the computation of the resistivity curves given the well trajectory and the geological model. The computed curves are compared to the measured curves so that a good match indicates the model is consistent. Otherwise, the model needs modifications and so the well trajectory. This field example also emphasizes the difficulties to geosteer a horizontal well due to unexpected situations such as irregular surfaces, subseismic faults, lithology variations and fingers. Additionally, resistivity modeling is used to improve and better understand the geological model. The procedure modifies the geological model in order to match real and measured curves. The results of this simulation are also applied to formation evaluation due to the computation of both horizontal and vertical resistivities.

Figure 1 shows a field example with both the build and reach sections of a horizontal well. The geological model was taken only from a pilot well due to the absence of offset wells and includes a massive turbidite sequence. The middle track displays the resistivity curves acquired in real time from a LWD tool. The upper track shows the computed curves based on the actual well path and the geological model shown in the lower track. The presence of a fluid contact and adjacent bed implies the navigation of the well must be close to the top of the reservoir. Point A is the target zone entry indicating the well was successfully landed in the pay zone. The measured resistivity increases from 2 to 15 Ohms. in agreement with the computed curves. At point B, resistivity drops to 7 Ohms suggesting the well path is getting close to the upper shale. At this moment, the earth model had to be refined in order to include the shale proximity. Also, the well was gently steered down so that, at point C, the resistivity is back to the reservoir value. At point D, the well unexpectedly built up angle and hit the upper shale. The measured resistivity dropped to the shale value and the well was steered down again. From point D to E, resistivity is building up to 15 ohms, indicating the well is gently returning to the pay zone. Finally, at point F, a sharp decrease in resistivity indicates the well drifted off course again due to an unexpected buildup angle. Additionally, the top of the reservoir is actually an erosional and undulating surface that eventually intercepts the well path. Thus, the geological model was constantly refined through inverse modeling to capture new geological and geometrical features observed during drilling. Then, the new model was forwarded and compared to the measured data for consistency. The results indicate a good match between real and measured curves, suggesting the model can be applied to future wells of the area.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil