Formation Evaluation From Logging on Cuttings

This paper presents an overview of a vast research project named Formation Evaluation 2000 that was undertaken by Agip SpA and was aimed at characterizing in real time the formations encountered during drilling by the mean of measurements on drill chips. To date, the project has demonstrated the feasibility to obtain representative values of the P and S wave velocities, rock strength and deformability, permeability, porosity, density, residual fluid content and saturation. Further work is underway in order to gain access to the pore size distribution, the thermal expansion and the conductivity of the rocks. The paper presents the methodology used systematically to assess such a feasibility and illustrates the results obtained to date during the various sequences of the research - i.e. primary design of the measure, laboratory tuning and field applicability. Furthermore, a series of field cases where these techniques were used are presented in order to highlight the industrial applications of such a package of measurements. Introduction In the oil industry, Formation Evaluation is traditionally performed after the drilling of the well by a series of techniques amongst which one can mention:core measurements which give direct indications but which are necessarily limited in space as it is often uneconomical to core continuously all the formations of interest;logs which give continuous measurements but which are often indirect - e.g. porosity from sonic logs;well tests of whatever nature - e.g. may they be for permeability determination or fracturation pressure, etc. - and which give large scale information about the rocks.In practice, the main drawback of such techniques is not so much technical than temporal in so far as they allow the characterization of the formations only after the end of the well whilst a while drilling evaluation would benefit many operations. For this reason, the industry has developed the Measurement While Drilling (MWD) and Logging While Drilling (LWD) techniques which aim at obtaining a real time formation evaluation. These techniques consist in inserting high technology sensors in the Bottom Hole Assembly and at performing and recording various measures on the formations soon after they have been discovered by the bit. Nevertheless, such techniques also suffer from various drawbacks which are listed below in a non-exhaustive manner: i. in the case when the measurement is sent directly from the bottom of the hole to the rig floor, the lag time for the availability of the information is only due to the distance between the bit and the sensor which may be several hours if the instantaneous Rate Of Penetration (ROP) is slow - e.g. < 2 m/h -; ii. in the case when the measurement is not sent to surface, the information only becomes available once the bit is pulled out of hole which may mean several days after the formations have been uncovered by the bit; iii. MWD and LWD tools are expensive high technology equipments which require a high degree of well stability to be run such as to minimize the risk of leaving them downhole because of a stick pipe problem. iv. finally, the interpretation of the measurements may be quite complicated as corrections have to be brought for various factors such as for example the vibrations of the drillstring in the case of the Sonic While Drilling. Because of the global situation described above, Agip decided to investigate the possible use of cuttings to perform quantitative physical determinations of the properties of the formations encountered by the well. As questions about the physical representativity of cuttings were raised very early in the project, it was decided to follow a two stage approach: