Abstract

Conoco implemented a novel well testing program in six in fill wells to evaluate the differential pressure depletion in the Dakota formation in a portion of the San Juan Basin in Rio Arriba County, New Mexico. The field program consisted of performing pre-fracture nitrogen slug tests or pressure buildup tests. Each of the three Dakota sands were evaluated separately, and in four wells, simultaneously to estimate permeability-thickness product, skin factor, reservoir pressure, and gas in place. The novel approach allowed for all sands to be tested quickly. The infill wells were drilled on 80-acre spacing among existing 160-acre spaced Dakota wells.

The Dakota formation is a series of tight, normal-pressured sands and is one of the main productive units in the San Juan Basin. The average reservoir pressure, as indicated by these tests, ranges from 2,500 psi to 2,900 psi. The original reservoir pressure was 3,200 psi. The post-fracture in-line production rates were excellent averaging between 350 to 900 Mscf/D after two months in-line. Single- and multi-well reservoir models were developed to predict the expected long-term performance from the infill wells.

This paper highlights the novel pre-fracture well testing program including the methodology utilized by Conoco to meet the challenging testing needs present in the Dakota formation in San Juan Basin. The testing approach of using either nitrogen slug tests or simultaneous, individual zone, buildup tests can be used in most tight gas sands consisting of multiple sands.

Introduction

The Cretaceous-aged Dakota formation is one of the primary producing intervals in the San Juan Basin located in northwestern New Mexico and southwestern Colorado. The Dakota has been a development objective and source of natural gas since the 1950's and continues to be a basin development objective today. Stratigraphically, the formation lays on top of the Jurassic-aged Morrison formation and commonly exists as a multi-layered, tight, naturally-fractured interbedded sand-shale sequence. The three main productive layers are the Two Wells, Upper Cubero, and Lower Cubero formations. Some wells produce from only these three Dakota sand packages, but more frequently, wells produce in combination with shallower Mesaverde horizons as a commingled completion.

In 1999, Conoco embarked on a test program to determine current reservoir pressure, permeability-thickness product, and skin factor in each of the three layers within the Dakota interval in the San Juan 28–7 Unit, located in Townships 28N–7W and 27N–7W, Rio Arriba County, New Mexico. The test program included drilling six 80–acre infill wells located in the north half of the San Juan 28–7 Unit as shown in the Fig. 1 base map. The primary objective of the test program was to measure pre-fracture pressure and gas flow rates from each Dakota interval for reservoir evaluation, while minimizing shut-in time. After pre-fracture testing, the entire Dakota interval was hydraulically fracture stimulated with a single-stage slickwater/proppant treatment, produced, and monitored.

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