Stratigraphic column of Ecuador’s Oriente Basin. The M1 reservoir consists of members of the Cretaceous Napo horizon. (Images courtesy of Repsol)

Over the last decade, continuous technical inroads have been made in drilling challenging reservoirs in Ecuador. All operators face high geological uncertainty and very active oil/water contacts that migrate upwards rapidly as fields are produced. In general, development wells in Ecuador have been vertical or of low-inclination, but these suffer from reduced length drainage intervals, low oil production and fast water breakthrough due to coning.

Horizontal drainholes minimize these effects. However, a more thorough understanding of the reservoir is needed, given the uncertainties that severely affect horizontal well placement. Early horizontal wells depended on simple real time measurements to steer them. With this technique a few of the horizontal drainholes were drilled out in part of the reservoir. Nevertheless, horizontal drilling was seen as the clear solution to improving reservoir productivity, so the company persisted in its efforts to find ways to improve well performance.

A challenging environment

The Block 16 field, operated by Repsol YPF is located in the Oriente basin of eastern Ecuador. The main oil producing reservoirs are located within Napo and Hollin formations. The Napo formation consists of four main producing intervals, the M1/M2 sands, the U sand bodies and the T sand (Figure 1). The M1 reservoir is characterized by high geological uncertainty as a result of its fluvial depositional environment. These sands usually have oil full to the base, except towards the borders of the structures, where an oil-water contact is present. Upper U are facies constituted by tidal-dominated estuarine quartzitic sands, with variable thickness. These sands are oil
saturated to the base, and the production is supported by a partial water drive, and rock and fluid expansion mechanisms. To minimize the uncertainties related to horizontal well placement within these sands, the execution of pilot wells was taken as a mandatory step before drilling the horizontal section.

Improving well placement

In December of 2006, Repsol YPF decided to geosteer a horizontal well using Schlumberger’s PeriScope bed boundary mapper system. Geosteering involves taking advantage of real-time data from logging-while-drilling tool systems to make steering decisions that place the well trajectory optimally within the target geology and maximize reservoir exposure.

Repsol YPF had good previous expertise in horizontal drilling. Nearly 80 horizontal wells were drilled in Ecuador using average gamma ray and resistivity measurements; however there was still room for improvement and risk minimization.

The PeriScope system has demonstrated its ability to deliver maximum reservoir coverage in drainholes as long as 7,000 ft (2,134 m) and with thicknesses as little as 5 ft (1.5 m) in all kinds of lithologies from sandstones to carbonates to coalbeds. The system’s ability to image reservoir boundaries as much as 21 ft (6.4 m) away from the borehole allows the geosteering team plenty of time to steer away from hazards even when drilling at high penetration rates.

Full speed ahead

To improve reservoir geometry identification in real time, a new solution was developed for the horizontal drilling campaign in Ecuador.

PeriScope’s deep-reading images combined with those of a shallow-reading azimuthal gamma ray measurement result in a new and stronger answer for well placement. This helps identify low rejection faults, small dip changes, thin layers, or low-contrast beds, not easily identified with deep directional images only. Schlumberger engineers figured a way to record and transmit both images in real-time to improve proactive geosteering decisions. Subsequent wells drilled using these measurements enabled detection of some features difficult to identify with deep imaging alone. By combining the measurements there was no need for a second tool system to be added to the bottomhole assembly (BHA). And since the two measurements are based on entirely different physical principles, they complement each other and add credence by confirming the presence of important features.

Pre-job planning was accomplished using Petrel seismic-to-simulation software to model the reservoir in 3-D. The model was then exported to the real-time geosteering software which is the core application for real-time well placement. This allows visualization of the well and the reservoir as drilling progresses. The inversion of the real-time electrical propagation resistivity measurements is presented in a “curtain diagram” that is enhanced by polar plots that indicate the apparent dip of overlying and underlying boundaries or fluid contacts. All data were transmitted to Repsol YPF offices where the multi-discipline core well placement team that included geoscientists, drilling engineers, and well placement engineers collaborated in the geosteering decisions.

Improved well placement

The test of the new technique was a success for geosteering, and showed good potential to improve well placement for future horizontal wells. Thus encouraged, the company used the new technique for the subsequent horizontal wells drilled with Schlumberger, placing an additional 15 wells through January of 2008. For all these wells, the preferred surface to be detected was the top of the reservoir.?However, the deep-imaging Periscope showed that in most cases this marker consisted of undulating or?variable shale-sand interfaces, making well placement as near as possible to the top very challenging.

The value of the real-time geosteering techniques using combined deep and detailed imaging can be illustrated with two examples. The first was projected to drain the M1-C sandstone in the IRO field. The objective was to drill a 1,000-ft (305-m) horizontal drain hole, keeping as close to the reservoir top as possible to provide access to valuable attic oil. The team set a goal of maintaining 9 ft (2.7 m) as the optimum proximity to the reservoir boundary.

The drainhole was drilled using a 61?8-in. polycrystalline diamond compact (PDC) bit. The 7-in. casing was landed in the reservoir about 5 ft (1.5 m) from the upper boundary. As drilling progressed, the tool showed the formation boundary to be dipping at a constant 1.2º, so it was easy to anticipate when to take the decision to drop hole angle to stay within the reservoir. When the boundary was four ft (1.2 m) away, the well was steered down to 89º (Figure 2). The real-time gamma ray image shows that the well barely touched the upper boundary as it steered away from it.

Drilling ahead, it was discovered that the reservoir top changed its dip to about 1º in the opposite direction from the borehole, so another steering decision was made, bringing the hole inclination to 91º, to follow the plane of the boundary. The reservoir top was mapped in real-time allowing the directional driller to stay on course with the undulations of the boundary for the entire length of the drainhole. In the M1-C well, the drainhole achieved 100% coverage, in the best quality part of the reservoir, away from the water contact and with formation resistivity above 90 ohm/m throughout. Not only was the well successfully placed, but it was drilled without the necessity of drilling a pilot hole, estimated to save the company about US $500,000.

Repsol YPF drilled more than 14,500 ft (4,422 m) in Ecuador using the PeriScope system. The average net pay in the 16 horizontal wells drilled with this technology for geosteering was 94%, having wells drilled 100% within the target, close to the reservoir top. Thus, geosteering has proven its ability to reduce the number of pilot wells needed, and provide access to valuable attic oil. Repsol plans to continue using PeriScope to optimize well placement for those wells with high geological uncertainty.