IDENTIFICATION OF NATURAL OPEN FRACTURES, INDUCED FRACTURES AND MATRIX PERMEABILITY IN CARBONATES WHILE DRILLING

Abstract

The economic feasibility of a well drilled in tight carbonates is extremely dependent on the level of fracture permeability; hard and dense carbonate formations may not be considered as net pay without the presence of fractures. The evaluation of fractures is a key to reservoir effectiveness characterization for well drilling, completion, development and stimulation of fractured reservoirs. While knowledge of the geological conditions and regional stress is helpful to estimate the characteristics of the natural fracture system in a given reservoir, the true extent of the natural open fracture system in any specific location is typically unknown. Several methods are available to the industry to identify natural fractures near the wellbore, including acoustic and resistivity image logs. In some cases, the poor-quality results of these techniques do not provide reliable information and such data cannot be available in all the wells. When minor downhole losses are accurately detected, it is possible to locate and characterize the natural open fractures intersected by the drill bit while drilling operations. The differential flow (Flow-out minus Flow-in) and the Active Volume System are continually monitored during drilling and integrated with drilling and hydraulic parameters. These readings are processed in a computer-based, data-acquisition system to form a compensated delta-flow signal that identifies the occurrence of downhole fluid losses. The differential flow is measured accurately through a dedicated Coriolis type flow-meter with a Limit Of Detection up to 10 l/min. By accurately detecting and measuring the downhole micro-losses instantaneously at the surface, the responses would be compared to predefined models for fracture characterization; that enables identification of different types of fractures (open natural, induced fractures). The system can detect very fine micro-fractures that might not be visible with wireline images; fracture density plots can then be created to highlight the fracture concentration along the well. Drilling deep wells in Kuwait is challenging due to high pressure, high-temperature formations, with the Bottom Hole Pressure of +15kpsi and Bottom Hole Temperature of +150 Centigrade degrees. In conventional surface systems, the loss detection relies on the Active Volume System and the Paddle type Flow-out sensor; however, these systems usually fail to identify the minor mud losses associated to open fractures. Especially for active pits with a big surface, it is almost impossible to identify few millimetres of mud level decrease and during fluid transfers, mud conditioning will make the job even more difficult to identify minor losses. With flow paddle type of sensors, the flow out information is not displayed as a calibrated value but rather as a percentage of full scale, which can be difficult to interpret. Instead, dedicated Coriolis type flowmeters properly installed, can identify flow rate changes accurately, regardless of any transfer of mud, water or diesel between pits. By applying this technique, it is possible to identify fractures while drilling in different types of wells, such as vertical, highly deviated and horizontal. The data were validated initially through core and image logs and further applied in next drilling campaigns.