A NEW PSEUDO-VELOCITY-BASED METHOD TO MITIGATE LWD IMAGE INTERPRETATION RISKS

Abstract

Borehole Imaging technology is often key for reservoir characterization and becomes more relevant when images are acquired while drilling to capture reservoir geology and petrophysical property distributions around the borehole. Logging While Drilling (LWD) high-resolution electrical/acoustics images of the borehole can resolve formation layers and heterogeneity down to 5mm (0.2in) scale and can detect response from far smaller features. This allows both, improved operational efficiency and better-informed drilling as well as shortening of the geological interpretation turn-around time from wireline logging time (days after drilling) to semi-real time (drilling time or hours after drilling). LWD high resolution images often suffer from the lack of direct downhole velocity measurements against the sensors. Depth tracking is on surface, referenced to the surface block movement. The imaging sensor acquiring data can be thousands of feet away from this surface reference. Imaging sensors on the bottom-hole assembly (BHA) are located not too far away from the drill bit. They are also subject to complex drilling-time motion such as tool whirling, stick and slip, vibration, mode coupling etc. This can make the downhole sensor movement dis-synchronized with the surface pipe depth increment. The Time-Depth conversion may accordingly get dis-synchronized to generate LWD depth image with missing features and distorted feature-integrity in depth. In severe conditions distorted image impacts real time image feature interpretation and leads to increased interpretation uncertainties. In this paper we investigate two main dis-synchronization problems using synthetic data: heave effect and BHA stick and slip effect. Pseudo velocity is computed from the surface measurement due to the lack of downhole sensor velocity direct measurement. In order to minimize heave effect, an advanced band-pass filter is proposed. The filter order is chosen in consistency with the sensor’s pseudo velocity behavior. Other properties of this advanced filter are also presented. In order to minimize the BHA stick and slip effect, pseudo velocity is analyzed as a delayed and minimized representative of the downhole sensor movement. A windowed-thresholding method is proposed to restore the compressed and stretched image features. Dip error analysis is performed by picking bed and fracture surface on the synthetic image data, before and after image distortion correction. The analysis results show a non-negligible effect on the accuracy of the true dip computed if the distortions are left un-corrected. Even in favorable logging conditions, the apparent dip error can contribute up to 50% of the total error. In this case, the image post-processing method proposed in this paper can not only improve the image quality but also reduce image interpretation uncertainties.