REAL-TIME DETECTION OF TAR IN CARBONATES USING LWD TRIPLE COMBO, NMR AND FORMATION TESTER IN HIGHLY-DEVIATED WELLS

摘要

Detection of tar, which is defined in this paper as the heavy oil phase that does not flow using standard production technologies, is critical for reservoir characterization, reserve calculations and well placement. Hydrocarbons that can not be produced must be classified as resources rather than reserves, and the distribution of tar must be accurately known for the proper placement water injectors and producers in heavy oil reservoirs. Tar must be detected as early as possible during the drilling of a well, in order to allow for the timely execution of necessary changes to the drilling program and to prevent costly remedial action, such as sidetracking. Early detection requires the deployment of LWD technologies for real-time or near real-time interpretation of the data. Lack of invasion observed in resistivity logs or borehole enlargement from the caliper log can be correlated to the presence of tar, the ideal tool string for tar detection includes triple-combo, NMR and the formation tester. The associated workflow utilizes a combination of tar indicators obtained from triplecombo and NMR logs. Once detected, mobility readings from the formation tester are then used to verify the presence of tar. The first tar indicator deployed is the well-known missing-porosity. A portion of the NMR signal in tar decays too fast to be detected by the logging tool, resulting in a deficit in NMR porosity when compared to total porosity obtained from conventional logs. The second and equally effective tar indicator is called the excess-bound-fluid, defined as the difference of bound fluid porosity from the NMR log and the bulk volume of water computed using conventional logs. Real-time LWD T_2 spectra, a recent development with significant future potential, can additionally be used to enhance the interpretation in resolving fluid-property related ambiguities, when available. Real-time tar characterization demands high accuracy from the porosity logs. In the clean carbonate formations typical of the Middle East, this requirement translates into the use of an 18-second wait-time to ensure complete polarization in the NMR log. The use of such a long wait-time impacts the ROP and vertical resolution of the NMR measurement, necessitating the availability of certain features in the hardware used. This paper discusses the petrophysical framework for a best-practice real-time tar detection workflow using LWD measurements. Results from the implementation of the methodology in two different carbonate fields in Saudi Arabia demonstrate that with proper technologies and robust interpretation algorithms in place, real-time tar identification can be done efficiently and accurately.