Over the last decades horizontal drilling has been performed on the StatoilHydro operated Troll field. During this time the total distance of horizontal wells within the reservoir has exceeded 1.600.000 feet, and Logging While Drilling (LWD) has been utilized to log gamma ray, resistivity, density and neutron porosity.
This tremendous amount of horizontal resistivity data acquired over time gives us an opportunity to assess how log response interpretation changes with time. When the first horizontal wells were drilled the field was in its virgin state and horizontal log interpretation was in its infancy. Over time the understanding of the horizontal log responses has improved. Tremendous changes in drilling technology and better knowledge of horizontal log responses and the impact on wellbore placement have led to an exceptional good production from the field. Over the last decade the Troll field has entered a more mature state, and log interpretation has become more challenging as the log responses are clearly affected by production. A second complicating factor is the variation in sand quality within the reservoir, from extremely high porosity and permeability to medium to high porosity and permeability. The third complicating factor is the presence of calcite cemented sandstone nodules which gives a very high resistivity reading in the near zero porosity rock.
This paper illustrates the steps necessitated in the horizontal log interpretation over time when the field is changing from virgin to mature, and discussing the factors having the greatest influence on the horizontal resistivity log responses. To improve our understanding of horizontal resistivity log responses, we examine facies across the field and the effects of fluid movements.
Recently, azimuthal propagation resistivity has been utilized on the Troll field. These measurements add a new dimension to horizontal resistivity log interpretation.
Located offshore on the Norwegian continental shelf, the Troll field is one of the world's largest gas fields, with a gas leg of up to 200 meters and only a thin oil leg. Nevertheless, Troll has over the last decades become one of largest oil producers on the Norwegian sector. Figure 1 shows the net oil and gas production from the Troll field.
The Troll field consists of two main structures, the Troll East and Troll West, both having a thin oil column below the gas cap, thus far only the Troll West structure with an oil leg of 11–26 m, has been found to be commercially viable. Oil production from the Troll field is based on a gas-cap drive mechanism.
A strong belief in the feasibility of developing the thin oil leg on Troll West, combined with enabling horizontal drilling technology, lead to the first successful horizontal production well being drilled on Troll West in 1989. In addition to the new horizontal drilling technology, state of the art LWD formation evaluation logging tools were utilized right from the start of drilling these horizontal wells.
1.600.000 feet of horizontal drilling and logging has been performed since the first horizontal well was drilled.
