fax 01-214-952-9435. ' AbstractThe introduction, a few years ago, of shear dipole sonic logs gave the industry the possibility to record high-quality shear and compressional slownesses in soft formations. Data sets were acquired and analyzed on Vp/Vs versus i1tc crossplots. Trends were identified in sands and shales and were matched with semi-empirical correlations based on the Gassmann formalism. These trends can be used to quality control shear logs and for quicklook lithology interpretation. The presence of gas in soft formations makes the interpretation more complicated as it can affect the sonic slownesses significantly, in particular the compressional. On the VpNs crossplot, gas-bearing formations clearly differentiate from liquid tilled formations. However, quantitative interpretation of the gas effect with the Gassmann equation gives deceptive results, although this model is successfully used in geophysics interpretation at a lower frequency. We indicate that the Gassmann model itself is not at fault. The responsibility is with the pore fluids mixture law used to compute the average fluid properties. We therefore propose a new empirical mixture law that better fits laboratory measurements and field observations. Using this revised model realistic gas trends can be identified on the VpNs crossplot. The model can be solved to evaluate gas volume from compressional and shear slownesses. Additionally, the effect of shaliness can be accounted for. The results agree well, in most instances, with flushed-zone saturation obtained from resistivity measurements and provide another opinion on gas volume. An additional product of the interpretation is to provide reliable values of dry-frame dynamic elastic constants of the rock for possible subsequent use in a rock mechanics evaluation.

Borehole to Surface Electromagnetic (BSEM) technology was conceived in the former Soviet Union and fine-tuned by the Chinese Bureau of Geophysical Prospecting (BGP). Saudi Aramco recently deployed the first BSEM pilot test outside of China (Marsala et al., 2011). This paper describes a new world first innovative electromagnetic borehole to surface survey in a well completed with multiple casings. The objective was to deploy a single BSEM survey to map the oil-water distributions in two separate reservoirs. This BSEM survey was conducted in a mature Saudi Arabian oil field composed of two main naturally fractured carbonate reservoirs, separated by a thick nonpermeable zone. The Upper reservoir is prolific, while the Lower reservoir is relatively tight and highly fractured. The reservoir pressure data from the early production period confirmed communication between the two reservoirs through several large scale fractures crossing the nonpermeable zone. In the Lower reservoir, well log observations show a variable oil-water distribution. No direct measurements of fluid saturations are available in the inter-well areas. The BSEM survey was designed to fill this data gap. In June 2012, a very challenging BSEM field acquisition was successfully completed with zero downtime and no accidents, obtaining very good data quality. Electromagnetic (EM) signals were transmitted at multiple frequencies from four source locations placed in a single vertical transmitting well that cross through both reservoirs and received by more than 1,000 surface stations, located in a grid at distances up to 3.5 kilometers away from the transmitting well. Multidisciplinary teamwork and independent peer reviews are undertaken to guarantee the optimal benefit from this pioneering technology. The business impact is to increase recovery by maximizing sweep efficiency and optimize well placements. Introduction A state-of-the-art Borehole to Surface Electromagnetic (BSEM) survey has recently been acquired in a giant mature oil field located in the Eastern Province of Saudi Arabia. The production from this field has been primarily from two fractured carbonate reservoirs, Upper and Lower, which are separated by a 500 ft thick, non-reservoir limestone formation. The Upper reservoir is prolific throughout the whole field and its high rate producers have been responsible for the majority of the historic field production. The Upper reservoir performance, including waterflood fronts, has been very predictable, which have made it easy to identify well targets and plan successful new development wells, sidetracks and other well remedial action based solely on well data like production performance, inflow profiles and saturation logs. The Lower reservoir is oil bearing only in the southern part of the field. This reservoir has low matrix permeability with well productivities and inflow profiles controlled mainly by a complex fracture system. A comprehensive Lower reservoir development drilling program is currently ongoing to augment the reservoir production. Due to the complex fracture system, the Lower reservoir development drilling program has been prone to unpredictable well fluid saturation results. Another element of uncertainty is the presence of several large near vertical fractures creating communication with the significantly more mature Upper reservoir. In structural positions where the basal part of Upper reservoir has been swept by water, these vertical fractures create pathways for water gravity dumping from Upper to Lower reservoir. A further complication is that the actual locations of these fracture communication pathways are not always known, which at times results in well logs showing unexpected water bearing fractures and water imbibed matrix.

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:

The first Borehole to Surface Electromagnetic (BSEM) pilot field survey in the Kingdom of Saudi Arabia (KSA) was successfully executed to identify oil and water bearing reservoir layers in a carbonate oilfield water injection zone. Maximizing recovery factor by means of detailed mapping of hydrocarbon accumulations in the reservoirs is a key requirement for oil producing companies. This mapping is done routinely by accurate measurements of fluid distribution at the wells' locations, but a knowledge gap exists in the inter-well volumes, where typically only density-based measurements are available (seismic and gravity). Such technologies are not always effective to discriminate and quantify the fluids in the porous space (especially when difference in fluid densities is small, such as oil and water). On the contrary, when high electrical resistivity contrasts exist between hydrocarbons and water, electromagnetic (EM) based technologies have the potential to map the distribution of the fluids and monitor their movement during the life of the field, hundreds of meters or kilometers away from the wellbores. The objective of a BSEM survey is to obtain fluid sensitive resistivity and induced polarization maps. These are based on an acquisition grid at the surface, a few kilometers around the EM transmitting well, which reveal oil and water bearing zones in the investigated reservoir layers. In this pilot field test, BSEM showed the potential to map water-front movements in an area of about 4km from the single well surveyed, evaluate the sweep efficiency, identify bypassed/ lagged oil zones, and eventually monitor the fluid displacements, if surveys are repeated over time. The data quality of the recorded signals is highly satisfactory. Fluid distribution maps obtained with BSEM are coherent with production data measured at the wells' locations, filling the knowledge gap of the interwells area.Three key R&D objectives for this BSEM pilot are achieved. Firstly, the capability to record at the surface EM signals generated in the reservoir, secondly, the capability of BSEM to discriminate between oil and water saturated reservoir zones, and finally obtain resistivity maps and a fluid distribution estimate plausible and coherent with the information obtained from well logs, crosswell EM, production data and reservoir models. In addition to reservoir monitoring, BSEM can be very useful in non-diagnosed areas like exploration fields for hydrocarbon exploitations.

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