What Is the Archie Water Saturation & Well Log Resistivity Calculator?
Archie’s equation is the foundational quantitative relationship in formation evaluation, enabling petrophysicists to determine the fraction of pore space filled with water (Sw) versus hydrocarbons (Shc).
Because hydrocarbons are electrical insulators while saline formation water conducts current, high rock resistivity correlates directly with oil and gas saturation.
How Does the Archie Water Saturation & Well Log Resistivity Calculator Work?
The formation factor F = a / ϕ^m models how rock mineral grains constrict electrical flow through water-filled pores.
The resistivity index I = Rt / Ro scales the added resistivity caused by non-conductive oil or gas displacing brine.
Archie Water Saturation & Well Log Resistivity Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Computes formation factor F = a/ϕ^m and 100% wet resistivity R₀ = F·Rw to determine brine and hydrocarbon fraction.
How to Use the Archie Water Saturation & Well Log Resistivity Calculator
- Obtain Rt from deep logging tools (e.g. AIT, HRLA) and Rw from water catalogs or SP logs.
- Input rock porosity from neutron-density logs.
- Check Bulk Volume Water (BVW = ϕ · Sw) to verify irreducible water saturation.
Step-by-Step Example Calculation
Clean Sandstone Hydrocarbon Zone
Input Values:
Understanding Your Result
Sw < 50% generally indicates potentially commercial hydrocarbon production.
Shc = 1 - Sw gives the oil/gas saturation fraction.
Constant BVW with depth indicates the reservoir is at irreducible water saturation (water-free production).
Factors That Affect the Result
- Shale/clay volume: Dispersed clay conducts electricity, causing Archie’s equation to overestimate water saturation (shaly-sand models like Waxman-Smits or Simandoux are required).
- Rock wettability: Oil-wet rocks exhibit higher saturation exponents (n > 2.5).
When Should You Use This Calculator?
- Evaluating openhole wireline and LWD logs for pay zone identification.
- Volumetric reserve calculations in oil and gas field appraisals.
Assumptions & Limitations
- Valid strictly for clean (clay-free) sandstone and carbonate formations with uniform brine salinity.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Petrophysical accuracy is typically within ±5% saturation units in clean formations.