When dealing with naturally fractured formations, the knowledge of the location and the permeability of the fractures intersecting the wellbore has a strong technical and economical impact upon drilling, production and reservoir management strategies. This paper presents a methodology which allows to perform a real time characterization of the conductive fractures intercepted by the bit while drilling. Such fractures are detected by monitoring continuously the mud losses at the rigsite using flow-meters measuring both the ingoing and the outgoing mud flow. The accuracy of such measurements is very high (mud losses as small as 20 liters can be monitored), and therefore also the smallest conductive fractures can be pinpointed.
A simple analytical model describing the mud invasion into a single fracture is used to invert the mud loss data in order to estimate the aperture and permeability of each fracture. This model has been validated with core measurements, and it is used in real time to perform a quick but sufficiently precise analysis.
The application of this technique to several field cases is also illustrated and discussed in terms of:
interpretation of the process of fracture plugging;
real time evaluation of the hydraulic aperture of the conductive fractures;
generation of a "secondary permeability log".
Moreover some operative implications are discussed, such as:
in the field of drilling, the muds, the lost circulation materials (LCM) and the cement plugs can be selected properly in order to avoid massive circulation losses and to minimize the damage due to mud invasion;
in the field of production engineering, the completion schemes and the stimulation operations can be optimized through the knowledge of the most conductive intervals;
in the field of reservoir engineering, a better modeling can be performed and a better exploitation of the naturally fractured reservoirs can be achieved.
Detecting mud gains and losses is a standard practice during drilling, as it provides the quickest warning sign of dangers due either to kicks or to massive circulation losses. Moreover, when drilling naturally fractured reservoirs, mud loss data provide one of the most effective means to assess the existence of conductive fractures intercepting the wellbore and therefore to identify potentially producing intervals.
The most used technique is monitoring the level of the mud tanks with floating sensors or acoustic reflectors and measuring the cumulative volume of mud lost over a period of time. In terms of formation characterization, the analysis of such measurements is generally restricted to the detection of the presence of natural conductive fractures within a certain interval and to the qualitative estimation of their conductivity on the basis of the cumulative volume of mud lost. However, they are not accurate enough to be inverted in order to characterize the hydraulic properties of the fractures causing the losses, in fact:
these measurements cannot discriminate the contribution of each single fracture;
they measure the cumulative volume lost within a certain interval, which depends not only on the conductivity of the intercepted fractures, but also on other factors, such as the rheological properties of the mud, the fracture density, the overbalance pressure, the plugging capacity of the mud, etc.;
in the very best situations their accuracy is about 0.5 bbls, and smaller losses (and, consequently, smaller fractures) cannot be detected.