This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 102745, "4D-Pressure Pilot To Steer Well Spacing in Tight Gas," by E. Quint, M. Singh, SPE, P. Huckabee, SPE, D. Brown, C.B. Brake, J. Bickley, and B. Johnston, SPE, Shell E&P Americas, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. Obtaining accurate formation pressures in microdarcy rock is a formidable challenge. However, these pressures provide the best data to identify unique reserves. They help to determine the drainage areas as well as appropriate well spacing for tight gas reservoirs. A 4D-pressure pilot was designed and installed to measure pressure drop at two observation wells equipped with pressure gauges. The pressure gauges enable pressure monitoring along the wellbore (in space) and over time, hence the name 4D-pressure pilot. Introduction The Pinedale field, in the Green River basin, Wyoming, is an example of a tight-reservoir gas accumulation. The field is a northwest/southeast-trending, doubly plunging, asymmetric anticline, 35 miles long and 6 miles wide, with the west flank bounded by a buried, high-angle, east-dipping thrust fault having as much as 660 ft displacement. The discovery well for the field was drilled in 1939. Sporadic drilling and evaluation of the field continued for almost 60 years. All early wells encountered significant gas shows but low production rates because of poor reservoir quality and limited completion intervals. Most of the wells in the Pinedale field have been on production for only a few years, and the drainage area has not been established. Therefore, the appropriate well spacing has not been determined. Most tight gas fields in the area started with 40-acre well spacing and were downspaced subsequently to 20-, 10-, or 5-acre spacing. Getting the well spacing and pattern correct early in the development is essential for cost-effective exploitation. In a layered model, the formation pressure at 10- and 20-acre offset locations declined by hundreds of psi if the sands were connected. Conversely, there was no pressure change if the sands were totally isolated. Therefore, a better test to determine well interference and optimal well spacing was to drill a pilot hole and observe the pressure rather than production changes in individual sand packages in one well while producing the other. To achieve optimal well spacing for a field, models were built and tested against the observed pressure measurements. If the model predicted pressure changes accurately, it could be used to optimize drainage patterns. The drilled wells were removed from the model, and the acreage was developed in several patterns, after which the pattern with the highest net present valve was chosen and implemented.